Laser component, laser generating apparatus, and optical coherence tomograph

ABSTRACT

A laser component includes multiple laser devices arranged in an array to include different emission wavelengths, and a driving unit that, while switching each of the laser devices along the array into a turn-ON-enabled state that enables the laser device to turn to an ON state, brings one of the laser devices in the turn-ON-enabled state into an ON state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2016-147878 filed Jul. 27, 2016.

BACKGROUND Technical Field

The present invention relates to a laser component, a laser generating apparatus, and an optical coherence tomograph.

SUMMARY

According to an aspect of the invention, there is provided a laser component including multiple laser devices arranged in an array to include different emission wavelengths, and a driving unit that, while switching each of the laser devices along the array into a turn-ON-enabled state that enables the laser device to turn to an ON state, brings one of the laser devices in the turn-ON-enabled state into an ON state.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein:

FIG. 1 illustrates an example of an optical coherence tomograph according to a first exemplary embodiment;

FIG. 2 illustrates an example of a tunable laser apparatus according to the first exemplary embodiment;

FIG. 3 is an equivalent circuit diagram of a laser component according to the first exemplary embodiment, and illustrates a signal generating circuit that supplies signals such as signals used to drive the laser component;

FIGS. 4A and 4B are respectively an exemplary plan layout view and an exemplary cross-sectional view of the laser component according to the first exemplary embodiment, of which FIG. 4A is a plan layout view of the laser component, and FIG. 4B is a cross-sectional view taken along a line IVB-IVB in FIG. 4A;

FIGS. 5A to 5C are detailed illustrations of an island formed by stacking a setting thyristor and a laser diode, of which FIG. 5A is an enlarged cross-sectional view, FIG. 5B is a cross-sectional view taken along a line VB-VB in FIG. 5A, and FIG. 5C is a cross-sectional view taken along a line VC-VC in FIG. 5A;

FIGS. 6A to 6C each illustrate the structure of a laser diode, of which FIG. 6A illustrates a distributed feedback (DFB) laser, FIG. 6B illustrates a distributed Bragg reflector (DBR) laser, and FIG. 6C illustrates a Fabry-Perot (FP) laser;

FIG. 7 illustrates the relationship between the emission wavelength of each laser diode and the pitch of diffraction gratings;

FIGS. 8A to 8C illustrate a stacked structure of a setting thyristor and a laser diode in further detail, of which FIG. 8A is a schematic energy band diagram of the stacked structure of the setting thyristor and the laser diode, FIG. 8B is an energy band diagram of a tunnel junction layer in reverse biased condition, and FIG. 8C illustrates the current-voltage characteristics of the tunnel junction layer;

FIG. 9 is a timing chart illustrating the operation of a laser generating unit;

FIGS. 10A to 10F illustrate a method for fabricating a laser component, of which FIG. 10A illustrates a semiconductor laminate forming step, FIG. 10B illustrates a step of forming an n-ohmic electrode, FIG. 10C illustrates an etching step for exposing a tunnel junction layer, FIG. 10D illustrates a step of forming the current-blocking portion of a current confinement layer, FIG. 10E illustrates an etching step for exposing a p-gate layer, and FIG. 10F illustrates a step of forming a p-ohmic electrode and a back electrode;

FIGS. 11A to 11C illustrate materials forming a metal-like-conductivity group III-V compound layer, of which FIG. 11A illustrates the band gap of InNAs with respect to the composition ratio x of InN, FIG. 11B illustrates the band gap of InNSb with respect to the composition ratio x of InN, and FIG. 11C illustrates the lattice constants of group VI elements and group III-V compounds with respect to band gap;

FIG. 12 illustrates a laser generating unit including multiple laser components;

FIG. 13 is an equivalent circuit diagram of a laser component according to a second exemplary embodiment, and illustrates a signal generating circuit that supplies signals such as signals used to drive the laser component;

FIG. 14 is a timing chart illustrating the operation of a laser generating unit according to the second exemplary embodiment;

FIG. 15 illustrates changes in the optical intensity of a laser diode with time;

FIGS. 16A and 16B illustrate the optical intensity of a laser diode, of which FIG. 16A illustrates optical intensity with respect to current, and FIG. 16B illustrates changes in optical intensity with respect to time;

FIG. 17 is an equivalent circuit diagram of a laser component, and illustrates a signal generating circuit that supplies signals such as signals used to drive the laser component;

FIG. 18 is a timing chart illustrating the operation of a laser generating unit according to a third second exemplary embodiment;

FIG. 19 is an equivalent circuit diagram illustrating the circuit configuration of a laser component according to a fourth exemplary embodiment, and a signal generating circuit;

FIG. 20 is a timing chart illustrating the operation of a laser generating unit according to the fourth second exemplary embodiment;

FIG. 21 is a detailed illustration of an island formed by stacking a setting thyristor and a vertical cavity surface emitting laser in a laser component according to a fifth exemplary embodiment; and

FIG. 22 is a detailed illustration of an island formed by stacking a setting thyristor and a vertical cavity surface emitting laser in a laser component according to a sixth exemplary embodiment.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The following description uses chemical symbols to represent chemical elements, such as Al for aluminum.

First Exemplary Embodiment

Optical Coherence Tomograph

FIG. 1 illustrates an example of an optical coherence tomograph according to a first exemplary embodiment.

The optical coherence tomograph includes a tunable laser apparatus 1, an optical beam splitter 2, a reference light mirror 3, and a photo-detector 4. The optical coherence tomograph applies a laser beam toward a living body 5 to obtain signal strength along the depth of the living body 5. The optical beam splitter 2 is an example of an optical splitting/combining unit, the reference light mirror 3 is an example of a reference-light reflecting unit, and the photo-detector 4 is an example of a photo-detection unit.

The tunable laser apparatus 1, the optical beam splitter 2, and the living body 5 are placed so as to be aligned in a straight line. The reference light mirror 3 and the photo-detector 4 are placed across the optical beam splitter 2 in a straight line orthogonal to the straight line in which the tunable laser apparatus 1, the optical beam splitter 2, and the living body 5 are aligned.

The distance LS from the optical beam splitter 2 to the living body 5 is set substantially equal to the distance LR from the optical beam splitter 2 to the reference light mirror 3.

The operation of the optical coherence tomograph will be described below in greater specificity.

The tunable laser apparatus 1 emits light of multiple frequencies (wavelengths). It is assumed in this case that the tunable laser apparatus 1 emits light of frequencies f1 and f2. The light emitted by the tunable laser apparatus 1 is split into two beams of light (beams a and b) by the optical beam splitter 2. The beam a passes through the optical beam splitter 2 and continues to travel in a straight line toward the living body 5. The beam a is reflected at a depth of penetration in the living body 5 that is determined by its wavelength, and then returns toward the optical beam splitter 2. It is assumed in this case that in the living body 5, light of the frequency f1 reaches a depth d1, and light of the frequency f2 reaches a depth d2.

Then, the beam a is reflected at 45° by the optical beam splitter 2, and travels toward the photo-detector 4.

The beam b is reflected at 45° by the optical beam splitter 2 to form reference light. Then, the beam b is reflected by the reference light mirror 3 and returns toward the optical beam splitter 2. Then, the beam b passes through the optical beam splitter 2, and continues to travel in a straight line toward the photo-detector 4.

Then, the two beams a and b that have been reflected back combine to create interference light, which enters the photo-detector 4. The interference light detected by the photo-detector 4 is subjected to a Fourier transform to yield a signal in the depth direction of the living body.

In the living body 5, the light of the frequency f1 provides a signal corresponding to the depth d1, and the light of the frequency f2 provides a signal corresponding to the depth d2.

Therefore, the wider the range of frequencies (wavelengths) (the broader the bandwidth), and the smaller the tunable range of wavelengths (f2−f1 in the example illustrated in FIG. 1), the greater the depth and resolution of imaging provided by the resulting signal. Further, if it is possible to switch frequencies (wavelengths) at high speed, this leads to shorter observation time.

Tunable Laser Apparatus 1

FIG. 2 illustrates the tunable laser apparatus 1 according to the first exemplary embodiment.

The tunable laser apparatus 1 includes a laser generating unit 10, a coupling unit 20, and an optical output unit 30. The laser generating unit 10, which is an example of a laser generating apparatus and a laser generator, includes multiple laser devices (e.g., laser diodes in the first exemplary embodiment) that emit laser beams of different wavelengths.

The coupling unit 20 guides laser beams emitted by multiple laser devices to the optical output unit 30. In FIG. 2, the coupling unit 20 is a diffraction grating with a shape that is curved in an actuate manner. Although the term “coupling unit” is generally used for a component that guides multiple beams of light to the optical output unit 30, a component is herein referred to as coupling unit even if the component guides a single beam of light to the optical output unit 30. The coupling unit 20 may be, other than a diffraction grating, any component that guides light to the optical output unit 30. For example, the coupling unit 20 may be a multi mode interference (MMI) waveguide or an arrayed waveguide grating (AWG).

The optical output unit 30 outputs light (a beam of light) from the tunable laser apparatus 1. In FIG. 2, the optical output unit 30 is an optical fiber. The optical output unit 30 may b, other than an optical fiber, any component that outputs light in a predetermined direction from the tunable laser apparatus 1.

Laser Generating Unit 10

The laser generating unit 10 includes a laser component C including laser diodes LD arranged in an array, and a signal generating circuit 110 that drives the laser component C.

FIG. 3 is an equivalent circuit diagram of the laser component C according to the first exemplary embodiment, and illustrates the signal generating circuit 110 that supplies signals such as signals used to drive the laser component C.

First, the configuration of the signal generating circuit 110 will be described.

The signal generating circuit 110, which is an example of a controller, includes a transfer signal generating unit 120 that transmits transfer signals φ1 and φ2 to the laser component C.

The signal generating circuit 110 also includes a lighting signal generating unit 140 that transmits a lighting signal φI to the laser component C.

Further, the signal generating circuit 110 includes a reference-potential supply unit 160 that supplies a reference potential Vsub, which serves as the reference for potential, to the laser component C, and a power-supply-potential supply unit 170 that supplies a power supply potential Vgk for driving the laser component C.

Next, the laser component C will be described.

The laser component C includes a light emitting unit 102 formed by a light emitting device (laser device) array made up of laser diodes LD1 to LD128 (to be referred to as laser diode LD when not distinguished from each other). In this case, the laser diodes LD1 to LD128 emit light of different wavelengths (emission wavelengths/oscillation wavelengths) λ1 to λ128. It is assumed that the spacing Δλ between the wavelengths λ1 to λ128 is uniform. The spacing Δλ may not be uniform.

The respective emission wavelengths of the laser diodes LD1 to LD128 may not all be different. It suffices that at least two different emission wavelengths be included.

Irrespective of whether the laser diodes LD1 to LD128 are arranged in increasing or decreasing order of their emission wavelengths, the emission wavelength may differ for every single laser diode LD, or may differ for every multiple number of laser diodes LD, for example, every two laser diodes LD or every three laser diodes LD. When it is stated in the following description that the emission wavelengths are in increasing or decreasing order along the array of the laser diodes LD, this refers to not only when the emission wavelength differs for every single laser diode LD but also when the emission wavelength differs for every multiple number of laser diodes LD.

The laser component C includes setting thyristors S1 to S128 (to be referred to as setting thyristor S when not distinguished from each other). Of the laser diodes LD1 to LD128 and the setting thyristors S1 to S128, the laser diode LD and the setting thyristor S that are assigned the same number are electrically connected in series.

As illustrated in FIG. 4B described later, the laser diodes LD are stacked over the corresponding setting thyristors S that are arranged in lines over a substrate 80.

The laser component C includes transfer thyristors T1 to T128 (to be referred to as transfer thyristor T when not distinguished from each other), which are arranged in lines in a manner similar to the laser diodes LD1 to LD128 and the setting thyristors S1 to S128.

Although the transfer thyristors T will be described as an example of transfer elements in the following, any circuit elements that are sequentially switched ON may be used. For example, circuit elements that combine components such as a shift register and multiple transistors may be used.

With each two of the transfer thyristors T1 to T128 paired in numerical order, the laser component C includes coupling diodes D1 to D127 (to be referred to as coupling diode D when not distinguished from each other) each located between each two paired transfer thyristors T.

Further, the laser component C includes power line resistors Rg1 to Rg128 (to be referred to as power line resistor Rg when not distinguished from each other).

The laser component C includes a single start diode SD. Further, the laser component C includes current-limiting resistors R1 and R2. The current-limiting resistors R1 and R2 are respectively disposed to prevent excessive current from flowing through transfer signal lines 72-1 and 72-2 described later on which the transfer signals φ1 and φ2 are respectively transmitted.

In this case, the setting thyristors S1 to S128, the transfer thyristors T1 to T128, the power line resistors Rg1 to Rg128, the coupling diodes D1 to D127, the start diode SD, and the current-limiting resistors R1 and R2 constitute a driving unit 101.

The laser diodes LD1 to LD128 of the light emitting unit 102, and the setting thyristors S1 to S128 and the transfer thyristors T1 to T128 of the driving unit 101 are arranged in numerical order from the left in FIG. 3. Further, the coupling diodes D1 to D127, and the power line resistors Rg1 to Rg128 are also arranged in numerical order from the left in FIG. 3.

The driving unit 101 and the light emitting unit 102 are arranged in this order from the top in FIG. 3.

In the first exemplary embodiment, the light emitting unit 102 includes 128 laser diodes LD, and the driving unit 101 includes 128 setting thyristors S, 128 transfer thyristors T, and 128 power line resistors Rg. The number of coupling diodes D is 127, which is one less than the number of transfer thyristors T.

The number of components such as the laser diodes LD is not limited to the above but may be any predetermined number. The number of transfer thyristors T may be greater than the number of laser diodes LD.

The laser diodes LD mentioned above are two-terminal semiconductor devices each including an anode terminal (anode) and a cathode terminal (cathode). The thyristors (the setting thyristors S and the transfer thyristors T) are semiconductor devices with three terminals including an anode terminal (anode), a gate terminal (the gate), and a cathode terminal (cathode). The coupling diodes D and the start diode SD are two-terminal semiconductor devices each including an anode terminal (anode) and a cathode terminal (cathode).

As will be described later, depending on the case, the laser diodes LD, the thyristors (the setting thyristors S and the transfer thyristors T), the coupling diodes D, and the start diode SD may not necessarily include an anode terminal, a gate terminal, or a cathode terminal that is constructed as an electrode. Accordingly, in the following description, the term “terminal” will be sometimes omitted and the notations in parentheses will be used instead.

Next, electrical connection of various elements in the laser component C will be described.

The respective anodes of the transfer thyristor T and the setting thyristor S are connected to the substrate 80 of the laser component C (anode-common configuration).

These anodes are supplied with the reference potential Vsub from the reference-potential supply unit 160 of the signal generating circuit 110 via a back electrode 91 (see FIG. 4B described later), which is a Vsub terminal disposed on the back of the substrate 80.

This connection represents an example of configuration when the substrate 80 is a p-type substrate. The polarity is reversed if an n-type substrate is used. If an intrinsic (i-type) substrate with no impurity doped is to be used, a terminal for supplying the reference potential Vsub is disposed at the side of the substrate 80 where the driving unit 101 and the light emitting unit 102 are located.

The cathodes of the odd-numbered transfer thyristors T1, T3, and so on are connected to the transfer signal line 72-1 along the array of the transfer thyristors T. The transfer signal line 72-1 is connected to a φ1 terminal via the current-limiting resistor R1. The transfer signal φ1 is transmitted to the φ1 terminal from the transfer signal generating unit 120 of the signal generating circuit 110.

The cathodes of the even-numbered transfer thyristors T2, T4, and so on are connected to the transfer signal line 72-2 along the array of the transfer thyristors T. The transfer signal line 72-2 is connected to a φ2 terminal via the current-limiting resistor R2. The transfer signal φ2 is transmitted to the φ2 terminal from the transfer signal generating unit 120 of the signal generating circuit 110.

The cathodes of the laser diodes LD1 to LD128 are connected to a lighting signal line 75. The lighting signal line 75 is connected to a φI terminal. The lighting signal φI is transmitted to the φI terminal from the lighting signal generating unit 140 of the signal generating circuit 110. The lighting signal φI supplies a current for lighting the laser diodes LD1 to LD128 to the laser diodes LD1 to LD128.

Respective gates Gt1 to Gt128 (to be referred to as gate Gt when not distinguished from each other) of the transfer thyristors T1 to T128 are connected to respective gates Gs1 to Gs128 (to be referred to as gate Gs when not distinguished from each other) of the same-numbered setting thyristors S1 to S128 in one-to-one correspondence. Thus, of the gates Gt1 to Gt128 and the gates Gs1 to Gs128, the gate Gt and the gate Gs that are assigned the same number are electrically at the same potential. Accordingly, for example, the notation “gate Gt1 (the gate Gs1)” will be used to indicate that these gates are at the same potential.

With each two of the respective gates Gt1 to Gt128 of the transfer thyristors T1 to T128 paired in numerical order, the coupling diodes D1 to D127 are each connected between each two paired gates Gt. That is, the coupling diodes D1 to D127 are each connected in series so as to be sandwiched between each two of the gates Gt1 to Gt128. As for the orientation of the coupling diode D1, the coupling diode D1 is connected such that current flows from the gate Gt1 to the gate Gt2. The same applies to the other coupling diodes D2 to D127.

The gate Gt (the gate Gs) of each of the transfer thyristors T is connected to a power supply line 71 via the power line resistor Rg provided in correspondence with the transfer thyristor T. The power supply line 71 is connected to a Vgk terminal. The Vgk terminal is supplied with the power supply potential Vgk from the power-supply-potential supply unit 170 of the signal generating circuit 110.

The gate Gt1 of the transfer thyristor T1 is connected to the cathode terminal of the start diode SD. The anode of the start diode SD is connected to the transfer signal line 72-2.

FIGS. 4A and 4B are respectively an exemplary plan layout view and an exemplary cross-sectional view of the laser component C according to the first exemplary embodiment. FIG. 4A is a plan layout view of the laser component C, and FIG. 4B is a cross-sectional view taken along a line IVB-IVB in FIG. 4A.

The illustration in FIG. 4A focuses on the portion of the laser component C corresponding to the laser diodes LD1 to LD4, the setting thyristors S1 to S4, and the transfer thyristors T1 to T4. The Vsub terminal (the back electrode 91) disposed on the back of the substrate 80 is depicted to be extended to the outside of the substrate 80.

FIG. 4B, which is a cross-section taken along a line IVB-IVB in FIG. 4A, depicts the following components from the bottom: the laser diode LD1/the setting thyristor S1, the transfer thyristor T1, the coupling diode D1, and the power line resistor Rg1. The laser diode LD1 and the setting thyristor S1 are stacked together.

In FIGS. 4A and 4B, major elements, terminals, and other components are designated by corresponding reference signs.

First, the cross-sectional structure of the laser component C will be described with reference to FIG. 4B.

A p-type anode layer 81 (p-anode layer 81), an n-type gate layer 82 (n-gate layer 82), a p-type gate layer 83 (p-gate layer 83), and an n-type cathode layer 84 (n-cathode layer 84) are disposed in the stated order over the p-type substrate 80 (substrate 80). The notations in parentheses will be used in the following description. The same applies to other cases.

A tunnel junction (tunnel diode) layer 85 is disposed over the n-cathode layer 84.

Further, a p-type anode layer 86 (p-anode layer 86), a light emitting layer 87, and an n-type cathode layer 88 (n-cathode layer 88) are disposed over the tunnel junction layer 85.

The p-anode layer 81, the n-gate layer 82, the p-gate layer 83, the n-cathode layer 84, the tunnel junction layer 85, the p-anode layer 86, the light emitting layer 87, and the n-cathode layer 88, which are semiconductor layers, are stacked in the stated order through epitaxial growth. To form multiple islands (islands 301, 302, 303, and so on described later) that are separated from each other, semiconductor layers between the islands are removed by etching (mesa etching). The p-anode layer 81 may or may not be separated. In FIG. 4B, a part of the p-anode layer 81 is separated in the thickness direction. The p-anode layer 81 may double as the substrate 80.

As illustrated in FIG. 4B, the laser component C is provided with a protective layer 90 made of a translucent insulating material disposed so as to cover the surfaces and lateral sides of the above-mentioned islands. These islands and wiring lines such as the power supply line 71, the transfer signal lines 72-1 and 72-2, and the lighting signal line 75 are connected via through-holes (indicated by circles in FIG. 4A) provided in the protective layer 90. In the following, a description will not be given of the protective layer 90 and the through-holes.

As illustrated in FIG. 4B, the back electrode 91, which serves as the Vsub terminal, is disposed on the back of the substrate 80.

The p-anode layer 81, the n-gate layer 82, the p-gate layer 83, and the n-cathode layer 84 are used to form components such as the setting thyristor S, the transfer thyristor T, the coupling diode D, and the power line resistor Rg (the setting thyristor S1, the transfer thyristor T1, the coupling diode D1, and the power line resistor Rg1 in FIG. 4B).

As used herein, the names of the p-anode layer 81, the n-gate layer 82, the p-gate layer 83, and the n-cathode layer 84 correspond to their functions (actions) when these layers form the setting thyristor S and the transfer thyristor T. That is, the p-anode layer 81 acts as an anode, the n-gate layer 82 and the p-gate layer 83 each act as a gate, and the n-cathode layer 84 acts as a cathode. As will be described later, these layers have different functions (actions) when these layers form the coupling diode D and the power line resistor Rg.

The p-anode layer 86, the light emitting layer 87, and the n-cathode layer 88 form the laser diode LD (the laser diode LD1 in FIG. 4B).

The p-anode layer 86 and the n-cathode layer 88 are named similarly as descried above. That is, the names of these layers correspond to their functions (actions) when these layers form the laser diode LD. That is, the p-anode layer 86 acts as an anode, and the n-cathode layer 88 acts as a cathode. The p-anode layer 86 and the n-cathode layer 88 function as cladding layers of the laser diode LD. Accordingly, the p-anode layer 86 and the n-cathode layer 88 will be sometimes respectively referred to as p-anode (cladding) layer 86 and n-cathode (cladding) layer 88.

The laser diode LD emits light along the surface of the substrate 80 (as indicated by an empty arrow in FIG. 4A). The laser diodes LD1, LD2, LD3, and so on emit light of different wavelengths λ1, λ2, λ3, and so on.

The side of the laser diode LD that emits light is cleaved except for a current-blocking portion β of a current confinement layer 86 b included in the p-anode layer 86.

As described below, some of the multiple islands may not include some of the multiple layers including the p-anode layer 81, the n-gate layer 82, the p-gate layer 83, the n-cathode layer 84, the tunnel junction layer 85, the p-anode layer 86, the light emitting layer 87, and the n-cathode layer 88. For example, the island 302 does not include a part or all of the tunnel junction layer 85, the p-anode layer 86, the light emitting layer 87, and the n-cathode layer 88.

Further, some of the multiple islands may not include a part of its constituent layer. For example, although the island 302 includes the p-anode layer 81, the n-gate layer 82, the p-gate layer 83, and the n-cathode layer 84, the island 302 includes only a part of the n-cathode layer 84.

Next, the plan layout of the laser component C will be described with reference to FIG. 4A.

The island 301 is provided with the setting thyristor S1 and the laser diode LD1. The island 302 is provided with the transfer thyristor T1 and the coupling diode D1. The island 303 is provided with the power line resistor Rg1. The island 304 is provided with the start diode SD. The island 305 is provided with the current-limiting resistor R1, and the island 306 is provided with the current-limiting resistor R2.

In the laser component C, multiple islands similar to the islands 301, 302, and 303 are formed in parallel. Like the islands 301, 302, and 303, these islands are provided with components such as the setting thyristors S2, S3, S4, and so on, the laser diodes LD2, LD3, LD4, and so on, the transfer thyristors T2, T3, T4, and so on, and the coupling diodes D2, D3, D4, and so on.

Now, the islands 301 to 306 will be described in detail with reference to FIGS. 4A and 4B.

As illustrated in FIG. 4A, the island 301 is provided with the setting thyristor S1 and the laser diode LD1.

The setting thyristor S1 is formed by the p-anode layer 81, the n-gate layer 82, the p-gate layer 83, and the n-cathode layer 84. A p-type ohmic electrode 331 (p-ohmic electrode 331), which is disposed over the p-gate layer 83 exposed by removing the n-cathode layer 88, the light emitting layer 87, the p-anode layer 86, the tunnel junction layer 85, and the n-cathode layer 84, is formed as a terminal that serves as the gate Gs1 (to be sometimes referred to as gate terminal Gs1).

The laser diode LD1 is formed by the p-anode layer 86, the light emitting layer 87, and the n-cathode layer 88. The laser diode LD1 is stacked over the n-cathode layer 84 of the setting thyristor S1 with the tunnel junction layer 85 interposed therebetween. An n-type ohmic electrode 321 (n-ohmic electrode 321) disposed over the n-cathode layer 88 (region 311) serves as the cathode terminal.

The p-anode layer 86 includes the current confinement layer 86 b (see FIG. 5A described later). The current confinement layer 86 b is provided to confine the current flowing through the laser diode LD to the central portion of the laser diode LD. In this regard, the peripheral portion of the laser diode LD contains many defects caused by mesa etching. Thus, non-radiative recombination tends to occur at such defects. Accordingly, the current confinement layer 86 is provided to ensure that the central portion of the laser diode LD serves as a current-passing portion α that readily allows current to flow, whereas the peripheral portion of the laser diode LD serves as a current-blocking portion β that does not readily allow current to flow. As indicated by the laser diode LD1 in FIG. 6A, the area inside the broken line is the current-passing portion α, and the area outside the broken line is the current-blocking portion β.

The current confinement layer 86 b will be described later.

The presence of the current confinement layer 86 b reduces electric power consumed by non-radiative recombination, leading to reduced power consumption and improved light extraction efficiency. Light extraction efficiency refers to the amount of light that can be extracted per unit supplied electric power.

The island 302 is provided with the transfer thyristor T1 and the coupling diode D1.

The transfer thyristor T1 is formed by the p-anode layer 81, the n-gate layer 82, the p-gate layer 83, and the n-cathode layer 84. That is, an n-ohmic electrode 323, which is disposed over the n-cathode layer 84 (a region 313) exposed by removing the n-cathode layer 88, the light emitting layer 87, the p-anode layer 86, and the tunnel junction layer 85, serves as the cathode electrode. In some instances, an n⁺⁺ layer 85 a of the tunnel junction layer 85 may not be removed, and the n-ohmic electrode 323 may be disposed over the n⁺⁺ layer 85 a of the tunnel junction layer 85. Further, a p-ohmic electrode 332, which is disposed over the p-gate layer 83 exposed by removing the n-cathode layer 84, is formed as a terminal serving as the gate Gt1 (to be sometimes referred to as gate terminal Gt1).

Likewise, the coupling diode D1 provided in the island 302 is formed by the p-gate layer 83 and the n-cathode layer 84. That is, an n-ohmic electrode 324, which is disposed over the n-cathode layer 84 (a region 314) exposed by removing the n-cathode layer 88, the light emitting layer 87, the p-anode layer 86, and the tunnel junction layer 85, serves as the cathode electrode. In some instances, the n⁺⁺ layer 85 a of the tunnel junction layer 85 may not be removed, and the n-ohmic electrode 324 may be disposed over the n⁺⁺ layer 85 a of the tunnel junction layer 85. Further, the p-ohmic electrode 332, which is disposed over the p-gate layer 83 exposed by removing the n-cathode layer 84, serves as the anode terminal. In this case, the anode terminal of the coupling diode D1 is the same as the gate Gt1 (the gate terminal Gt1).

The power line resistor Rg1 provided in the island 303 is formed by the p-gate layer 83. The power line resistor Rg1 is a resistor formed by the portion of the p-gate layer 83 between a p-ohmic electrode 333 and a p-ohmic electrode 334, which are disposed over the p-gate layer 83 exposed by removing the n-cathode layer 88, the light emitting layer 87, the p-anode layer 86, the tunnel junction layer 85, and the n-cathode layer 84.

The start diode SD provided in the island 304 is formed by the p-gate layer 83 and the n-cathode layer 84. That is, an n-ohmic electrode 325, which is disposed over the n-cathode layer 84 (a region 315) exposed by removing the n-cathode layer 88, the light emitting layer 87, the p-anode layer 86, and the tunnel junction layer 85, serves as the cathode electrode. In some instances, the n⁺⁺ layer 85 a of the tunnel junction layer 85 may not be removed, and the n-ohmic electrode 325 may be disposed over the n⁺⁺ layer 85 a of the tunnel junction layer 85. Further, a p-ohmic electrode 335, which is disposed over the p-gate layer 83 exposed by removing the n-cathode layer 84, serves as the anode terminal.

The current-limiting resistor R1 provided in the island 305, and the current-limiting resistor R2 provided in the island 306 are disposed in the same manner as with the power line resistor Rg1 provided in the island 303. Each of the current-limiting resistor R1 and the current-limiting resistor R2 is a resistor formed by the portion of the p-gate layer 83 between two p-ohmic electrodes (no reference signs attached).

FIG. 4A illustrates the connection between various elements.

The lighting signal line 75 includes a trunk portion 75 a and multiple branch portions 75 b. The trunk portion 75 a is disposed to extend in the direction in which the laser diodes LD are arranged in lines. The branch portions 75 b are branched out from the trunk portion 75 a, and connected to the n-ohmic electrode 321, which is the cathode terminal of the laser diode LD1 provided in the island 301. The same applies to the cathode terminals of the other laser diodes LD.

The lighting signal line 75 is connected to the φI terminal disposed near the laser diode LD1.

The transfer signal line 72-1 is connected to the n-ohmic electrode 323, which is the cathode terminal of the transfer thyristor T1 provided in the island 302. The cathode terminals of the other odd-numbered transfer thyristors T provided in islands similar to the island 302 are also connected with the transfer signal line 72-1. The transfer signal line 72-1 is connected to the φ1 terminal via the current-limiting resistor R1 provided in the island 305.

The transfer signal line 72-2 is connected to an ohmic electrode (no reference sign attached) that serves as the cathode terminal of each of the even-numbered transfer thyristors T provided in islands to which no reference signs are attached. The transfer signal line 72-2 is connected to the φ2 terminal via the current-limiting resistor R2 provided in the island 306.

The power supply line 71 is connected to the p-ohmic electrode 334, which is one terminal of the power line resistor Rg1 provided in the island 303. Likewise, one terminal of each of the other power line resistors Rg is connected to the power supply line 71. The power supply line 71 is connected to the Vgk terminal.

The p-ohmic electrode 331 (the gate terminal Gs1) of the laser diode LD1 provided in the island 301 is connected to the p-ohmic electrode 332 (the gate terminal Gt1) of the island 302 via a connection wiring line 76.

The p-ohmic electrode 332 (the gate terminal Gt1) is connected to the p-ohmic electrode 333 (the other terminal of the power line resistor Rg1) of the island 303 via a connection wiring line 77.

The n-ohmic electrode 324 (the cathode terminal of the coupling diode D1) provided in the island 302 is connected to a p-type ohmic electrode (no reference sign attached), which is the gate terminal Gt2 of the adjacent transfer thyristor T2, via a connection wiring line 79.

Although not described here, the same as described above applies to components such as the other laser diodes LD, setting thyristors S, transfer thyristors T, and coupling diodes D.

The p-ohmic electrode 332 (the gate terminal Gt1) of the island 302 is connected to the n-ohmic electrode 325 (the cathode terminal of the start diode SD) provided in the island 304 via a connection wiring line 78. The p-ohmic electrode 335 (the anode terminal of the start diode SD) is connected to the transfer signal line 72-2.

The above-mentioned connection and configuration are employed when the substrate 80 used is a p-type substrate, and the polarity is reversed if an n-type substrate is used. If an i-type substrate is used, a terminal connected to the reference-potential supply unit 160 that supplies the reference potential Vsub is provided at the side of the substrate where the driving unit 101 and the light emitting unit 102 are located. The connection and configuration in this case are the same as either those employed when a p-type substrate is used or those employed when an n-type substrate is used.

Stacked Structure of Setting Thyristor S and Laser Diode LD

FIGS. 5A to 5C are detailed illustrations of the island 301 formed by stacking the setting thyristor S and the laser diode LD. FIG. 5A is an enlarged cross-sectional view, FIG. 5B is a cross-sectional view taken along a line VB-VB in FIG. 5A, and FIG. 5C is a cross-sectional view taken along a line VC-VC in FIG. 5A. The protective layer 90 is not illustrated in FIGS. 5A to 5C. The same applies hereinafter. FIG. 5A represents a cross-section of the island 301 illustrated in FIGS. 4A and 4B, as viewed in the −y direction in FIG. 4A. Because the p-ohmic electrode 331 becomes invisible in this state, the portion corresponding to the p-ohmic electrode 331 is viewed from the −x direction in FIG. 4A. The same applies hereinafter. FIGS. 5B and 5C are cross-sectional views taken in the y direction.

As described above, the laser diode LD is stacked over the setting thyristor S with the tunnel junction layer 85 interposed therebetween. That is, the setting thyristor S and the laser diode LD are electrically connected in series.

The setting thyristor S is formed by the p-anode layer 81, the n-gate layer 82, the p-gate layer 83, and the n-cathode layer 84. That is, the setting thyristor S has a four-layer, p-n-p-n structure.

The tunnel junction layer 85 includes the n⁺⁺ layer 85 a doped with a high concentration of n-type impurities (dopants), and a p⁺⁺ layer 85 b doped with a high concentration of p-type impurities.

The laser diode LD includes the p-anode layer 86, the light emitting layer 87, and the n-cathode layer 88.

The light emitting layer 87 has a quantum well structure formed by an alternate stack of well and barrier layers. The light emitting layer 87 may be an intrinsic (i-type) layer with no impurity doped. The light emitting layer 87 may have a structure other than a quantum well structure, for example, a quantum line (quantum wire) or quantum box (quantum dot) structure.

The p-anode layer 86 is made up of the following stack of layers: a lower p-anode layer 86 a, the current confinement layer 86 b, and an upper p-anode layer 86 c. The current confinement layer 86 b includes the current-passing portion α and the current-blocking portion β. As illustrated in FIG. 4A, the current-passing portion α is located in the central portion of the laser diode LD, whereas the current-blocking portion β is located in the peripheral portion of the laser diode LD.

Further, the n-cathode layer 88 includes a lower n-cathode layer 88 a and an upper diffraction grating layer (diffraction grating layer) 88 b. The upper diffraction grating layer 88 b, which is a portion with diffraction gratings (banded irregularities) formed at predetermined pitches, is contiguous with the lower n-cathode layer 88 a. That is, first, the n-cathode layer 88 is stacked, and then its surface is formed into a banded pattern. The portion of the n-cathode layer 88 that has a diffraction grating pattern formed in this way serves as the upper diffraction grating layer 88 b, and the portion of the n-cathode layer 88 that has no diffraction grating pattern serves as the lower n-cathode layer 88 a. The pitch of the diffraction gratings provided in the upper diffraction grating layer 88 b determines the wavelength emitted (oscillated) by the laser diode LD.

The diffraction grating layer may not necessarily be provided in the location mentioned above. For instance, a part of the light emitting layer 87 may serve as the diffraction grating layer, or a part of the lower p-anode layer 86 a or the upper p-anode layer 86 c in the p-anode layer 86 may serve as the diffraction grating layer.

As illustrated in FIG. 5C, light is emitted from a region (light emitting region) corresponding to the light emitting layer 87 located below the n-ohmic electrode 321, which is disposed over the upper diffraction grating layer 88 b.

FIGS. 6A to 6C each illustrate the structure of the laser diode LD. FIG. 6A illustrates a distributed feedback (DFB) laser, FIG. 6B illustrates a distributed Bragg reflector (DBR) laser, and FIG. 6C illustrates a Fabry-Perot (FP) laser.

Like the laser component C illustrated in FIGS. 5A, 5B, and 5C, the DFB laser illustrated in FIG. 6A has diffraction gratings disposed over the light emitting region below the n-ohmic electrode 321. With a DBF laser, the wavelength to be emitted (emission wavelength) is selected by the diffraction grating, and a single wavelength of light is emitted. The DBR laser illustrated in FIG. 6B has diffraction gratings outside the light emitting region located below the n-ohmic electrode 321. With such a DBR laser as well, the wavelength to be emitted (emission wavelength) is selected by the diffraction grating, and a single wavelength of light is emitted. In contrast, with the FP laser illustrated in FIG. 6C, laser oscillation is achieved by confining light between reflection surfaces created by, for example, cleaving of a crystal. The FP laser has no diffraction grating, and hence emits multiple wavelengths of light.

It is desirable to use a DFB laser or a DBR laser for the laser component C according to the first exemplary embodiment. The laser diodes LD illustrated in FIGS. 5A, 5B, and 5C are DFB lasers.

FIG. 7 illustrates the relationship between the emission wavelength of each laser diode LD and the pitch of diffraction gratings.

As the pitch of diffraction gratings in the upper diffraction grating layer 88 b is increased in the order of the laser diodes LD1, LD2, LD3, and so on, the wavelengths λ1, λ2, λ3, and so on emitted by the corresponding laser diodes LD increase in this order.

Tunnel Junction Layer 85

FIGS. 8A to 8C illustrate the stacked structure of the setting thyristor S and the laser diode LD in further detail. FIG. 8A is a schematic energy band diagram of the stacked structure of the setting thyristor S and the laser diode LD, FIG. 8B is an energy band diagram of the tunnel junction layer 85 in reverse biased condition, and FIG. 8C illustrates the current-voltage characteristics of the tunnel junction layer 85.

As illustrated in the energy band diagram of FIG. 8A, when a voltage is applied between the n-ohmic electrode 321 and the back electrode 91 illustrated in FIG. 5A such that the laser diode LD and the setting thyristor S become forward biased, the portion between the n⁺⁺ layer 85 a and the p⁺⁺ layer 85 b of the tunnel junction layer 85 becomes reverse biased.

The tunnel junction layer 85 (tunnel junction) represents a junction between the n⁺⁺ layer 85 a doped with a high concentration of n-type impurities, and the p⁺⁺ layer 85 b doped with a high concentration of p-type impurities. Hence, the depletion region is narrow, and upon application of a forward bias, electrons tunnel through from the conduction band on the side of the n⁺⁺ layer 85 a into the valence band on the side of the p⁺⁺ layer 85 b. This gives rise to appearance of negative resistance characteristics.

In contrast, as illustrated in FIG. 8B, upon application of a reverse bias (−V) to the tunnel junction layer 85 (tunnel junction), the potential Ev of the valence band on the side of the p⁺⁺ layer 85 b becomes higher than the potential Ec of the conduction band on the side of the n⁺⁺ layer 85 a. Then, electrons tunnel through from the valence band on the side of the p⁺⁺ layer 85 b into the conduction band on the side of the n⁺⁺ layer 85 a. Such tunneling of electrons occurs more readily as the applied reverse bias voltage (−V) increases. That is, as illustrated in FIG. 8C, the tunnel junction layer 85 (tunnel junction) readily conducts current in reverse biased condition.

Consequently, as illustrated in FIG. 8A, when the setting thyristor S turns ON, current flows between the setting thyristor S and the laser diode LD even when the tunnel junction layer 85 is under reverse bias. This causes the laser diode LD to turn to the ON (lit) state.

In this case, the setting thyristor S is enabled to turn to the ON state when the transfer thyristor T connected with the setting thyristor S turns ON and shifts to the ON state. Then, when the lighting signal φI goes to “Lo” described later, the setting thyristor S turns ON and shifts to the ON state, and causes (sets) the laser diode LD to light up. Hence, the name “setting thyristor” is used herein.

Thyristors

Next, the basic operation of thyristors (the transfer thyristor T and the setting thyristor S) will be described. As described above, a thyristor is a semiconductor device with the following three terminals: an anode terminal (anode), a cathode terminal (cathode), and a gate terminal (the gate). The thyristor is formed by, for example, stacking the following semiconductor layers made of GaAs, GaAlAs, AlAs, or other materials over the substrate 80: p-type semiconductor layers (the p-anode layer 81 and the p-gate layer 83), and n-type semiconductor layers (the n-gate layer 82 and the n-cathode layer 84). That is, the thyristor has a p-n-p-n structure. In this case, it is assumed, for example, that the forward potential (diffusion potential) Vd of the p-n junction formed by a p-type semiconductor layer and an n-type semiconductor layer is 1.5 V.

The following description assumes, for example, that the reference potential Vsub supplied to the back electrode 91 (see FIGS. 4B and 5A), which is the Vsub terminal, is a high-level potential (to be labeled “H” hereinafter) of 0 V, and the power supply potential Vgk supplied to the Vgk terminal is a low-level potential (to be labeled “L” hereinafter) of −3.3 V.

The anode of each thyristor is at the reference potential Vsub (“H” (0 V)) supplied to the back electrode 91.

A thyristor in its OFF state with no current flowing between the anode and the cathode goes to the ON state (turns ON) when a potential lower than (a negative potential larger in absolute value than) a threshold voltage is applied to the cathode. In this regard, the threshold voltage of the thyristor has a value equal to the potential at the gate minus the forward potential Vd (1.5 V) of the p-n junction.

Once turned ON, the potential at the gate of the thyristor becomes close to the potential of the anode. Since the anode is set to the reference potential Vsub (“H” (0 V)) in this case, the potential of the gate becomes 0 V (“H”). The cathode of the thyristor in its ON state is at a potential close to a value that is equal to the potential of the anode minus the forward potential Vd (1.5 V) of the p-n junction. Since the anode is set to the reference potential Vsub (“H” (0 V)) in this case, the cathode of the thyristor in the ON state is at a potential close to −1.5 V (a negative potential larger than 1.5 V in absolute value). The potential of the cathode is set in relation to the power supply that supplies current to the thyristor that is in the ON state.

A thyristor in its ON state goes to the OFF state (turns OFF) when the cathode is placed at a potential (a negative potential that is small in absolute value, 0 V, or a positive potential) higher than a potential required to maintain the ON state (a potential close to −1.5 V mentioned above).

When a potential lower than (a negative potential larger in absolute value than) a potential required to maintain the ON state is continuously applied to the cathode of a thyristor in its ON state to supply a current (holding current) that allows the thyristor to maintain the ON state, the thyristor maintains its ON state.

The setting thyristor S is stacked together and electrically connected in series with the laser diode LD. Hence, the voltage applied to the cathode (the n-cathode layer 84) of the setting thyristor S has a value equal to the potential of the lighting signal φI divided between the setting thyristor S and the laser diode LD. The following description assumes that the voltage applied to the laser diode LD is −1.7 V. The following description also assumes that when the setting thyristor S is in its OFF state, −3.3 V is applied to the setting thyristor S. That is, it is assumed that the lighting signal φI (“Lo”) applied to light the laser diode LD is −5 V.

The voltage applied to the laser diode LD is to be varied with the emission wavelength or light intensity. This may be accomplished by adjusting the voltage (“Lo”) of the lighting signal φI.

Since a thyristor is formed by a semiconductor such as GaAs, in some cases, light emission occurs between the n-gate layer 82 and the p-gate layer 83 while the thyristor is in the ON state. The amount of light emitted by a thyristor is determined by the area of the cathode and the current passed between the cathode and the anode. Accordingly, for cases where light emitted from the thyristor is not utilized, as is the case with the transfer thyristor T, for example, such unnecessary light may be minimized by reducing the area of the cathode, blocking light with an electrode (the n-ohmic electrode 323 of the transfer thyristor T1), or other methods.

Operation of Laser Generating Unit 10

Next, operation of the laser generating unit 10 will be described.

Timing Chart

FIG. 9 is a timing chart illustrating the operation of the laser generating unit 10.

FIG. 9 is a timing chart illustrating control of lighting (light emission) (to be referred to as lighting control) of five laser diodes LD, the laser diodes LD1 to LD5, of the laser component C. In FIG. 9, the laser diodes LD1, LD2, LD3, and so on of the laser component C are lit up in this order.

It is assumed that in FIG. 9, time passes in alphabetical order from time a to time k. It is assumed that times a, b, c, and so on in the timing chart differ for each timing chart.

The laser diodes LD1, LD2, LD3, and LD4 are caused to light up (lighting-controlled) respectively in periods U(1), U(2), U(3), and U(4). The laser diodes LD that are numbered 5 or higher are subsequently lighting-controlled in the same manner.

It is assumed in this case that the periods U(1), U(2), U(3), and so on are of the same length, and will be referred to as period U when not distinguished from each other.

Each of the transfer signals φ1 and φ2, which are respectively transmitted to the φ1 and φ2 terminals (see FIG. 3 and FIG. 4A) of the laser component C from the transfer signal generating unit 120 of the signal generating circuit 110, is a signal having two levels of potentials, “H” (0 V) and “L” (−3.3 V). Each of the transfer signals φ1 and φ2 repeats its waveforms every two consecutive periods U (e.g., the period U(1) and the period U(2)).

In the following description, “H” (0 V) and “L” (−3.3 V) will be sometimes abbreviated as “H” and “L”, respectively.

The transfer signal φ1 shifts from “H” (0 V) to “L” (−3.3 V) at the start time b of the period U(1), and shifts from “L” to “H” at time f. Then, at the end time i of the period U(2), the transfer signal φ1 shifts from “H” to “L”.

The transfer signal φ2, which is at “H” (0 V) at the start time b of the period U(1), shifts from “H” (0 V) to “L” (−3.3 V) at time e. Then, the transfer signal φ2 shifts from “L” to “H” at the end time i of the period U(2).

A comparison between the transfer signal φ1 and the transfer signal φ2 reveals that the transfer signal φ2 corresponds to the transfer signal φ1 shifted backward by the period U on the time axis. For the transfer signal φ2, the waveform indicated by a broken line in the period U(1) and the waveform in the period U(2) are repeated in the period U(3) and onward. The waveform of the transfer signal φ2 in the period U(1) differs from the waveform in the period U(3) and onward because the period U(1) corresponds to the period in which the laser generating unit 10 starts its operation.

As will be described later, the pair of the transfer signals φ1 and φ2 causes the ON state to be transferred (propagated) in the numerical order of the transfer thyristors T. As a result, the laser diode LD having the same number as the transfer thyristor T that is in the ON state is specified (selected) as the laser diode LD that is to be lighting-controlled.

Next, the lighting signal φI transmitted to the φI terminal of the laser component C from the lighting signal generating unit 140 of the signal generating circuit 110 will be described. The lighting signal φI has two levels of potentials, “H” (0 V) and “Lo” (−5 V).

In the following, the lighting signal φI will be described with respect to the period U(1) in which the laser diode LD1 of the laser component C is lighting-controlled. The lighting signal φI, which is at “H” (0 V) at the start time b of the period U(1), shifts from “H” (0 V) to “L” (−5 V) at time c. Then, the lighting signal φI shifts from “Lo” to “H” at time d, and stays at “H” at time e.

Now, with reference also to FIG. 3, the operation of the laser component C will be described with reference to the timing chart illustrated in FIG. 9. The periods U(1) and U(2) in which the laser diodes LD1 and LD2 are respectively lighting-controlled will be described below.

Time a

At time a, the reference-potential supply unit 160 of the signal generating circuit 110 sets the reference potential Vsub to “H” (0 V). The power-supply-potential supply unit 170 sets the power supply potential Vgk to “L” (−3.3 V). Then, the Vsub terminal of the laser component goes to “H”. Likewise, the Vgk terminal of the laser generating unit 10 goes to “L”. Thus, the back electrode 91 of the laser component C goes to “H”, and the power supply line 71 of the laser component C goes to “L” (see FIG. 3).

Then, the transfer signal generating unit 120 of the signal generating circuit 110 sets each of the transfer signals φ1 and φ2 to “H” (0 V). This causes the φ1 and φ2 terminals of the laser component C to become “H”. At this time, the potential of the transfer signal line 72-1 connected to the φ1 terminal via the current-limiting resistor R1 also becomes “H”, and the potential of the transfer signal line 72-2 connected to the φ2 terminal via the current-limiting resistor R2 also becomes “H”.

Further, the lighting signal generating unit 140 of the signal generating circuit 110 sets the lighting signal φI to “H” (0 V). As a result, the φI terminal of the laser component C goes to “H” via the current-limiting resistor RI, and the lighting signal line 75 connected to the φI terminal also goes to “H” (0 V).

Since the respective anode terminals of the transfer thyristor T and the setting thyristor S are connected to the back electrode 91, these anode terminals are set to “H”.

The respective cathodes of the odd-numbered transfer thyristors T1, T3, T5, and so on, which are connected to the transfer signal line 72-1, are set to “H” (0 V). The respective cathodes of the even-numbered transfer thyristors T2, T4, T6, and so on, which are connected to the transfer signal line 72-2, are set to “H”. Since the anode and cathode of each of these transfer thyristors T are both “H”, these transfer thyristors T are in their OFF state.

The cathode terminal of the laser diode LD is connected to the lighting signal line 75 that is at “H” (0 V). In this regard, the laser diode LD and the setting thyristor S are connected in series via the tunnel junction layer 85. Since the cathode of the laser diode LD is “H”, and the anode of the setting thyristor S is “H”, the laser diode LD and the setting thyristor S are in their OFF state.

The gate Gt1 is connected to the cathode of the start diode SD as described above. The gate Gt1 is connected to the power supply line 71 that is at the power supply potential Vgk (“L” (−3.3 V)), via the power line resistor Rg1. The anode terminal of the start diode SD is connected to the transfer signal line 72-2, and connected to the 02 terminal, which is at “H” (0 V), via the current-limiting resistor R2. Thus, the start diode SD is in forward bias, and the cathode (the gate Gt1) of the start diode SD is at a potential (−1.5 V) equal to the potential at the anode of the start diode SD (“H” (0 V)) minus the forward potential Vd (1.5 V) of the p-n junction. When the gate Gt1 becomes −1.5 V, the coupling diode D1 becomes forward biased because its anode (the gate Gt1) is at −1.5 V and its cathode is connected to the power supply line 71 (“L” (−3.3 V)) via the power line resistor Rg2. Thus, the potential of the gate Gt2 becomes −3 V, which is a value equal to the potential (−1.5 V) of the gate Gt1 minus the forward potential Vd (1.5 V) of the p-n junction. However, the effect of the anode of the start diode SD being “H” (0 V) does not extend to the gates Gt that are numbered 3 or higher, and the potential of each of these gates Gt is at “L” (−3.3 V), which is the potential of the power supply line 71.

Since the gate Gt is the same as the gate Gs, the gate Gs is at the same potential as the gate Gt. Thus, the threshold voltage of the transfer thyristor T and the setting thyristor S has a value equal to the potential at the gate Gt or Gs minus the forward potential Vd (1.5 V) of the p-n junction. That is, the threshold voltage of the transfer thyristor T1 and the setting thyristor S1 is −3 V, the threshold voltage of the transfer thyristor T2 and the setting thyristor S2 is −4.5 V, and the threshold voltage of the transfer thyristor T and the setting thyristor S that are numbered 3 or higher is −4.8 V.

Time b

At time b illustrated in FIG. 9, the transfer signal φ1 shifts from “H” (0 V) to “L” (−3.3 V). This causes the laser component C to start its operation.

When the transfer signal φ1 shifts from “H” to “L”, the potential of the transfer signal line 72-1 shifts from “H” (0 V) to “L” (−3.3 V) via the φ1 terminal and the current-limiting resistor R1. Then, the transfer thyristor T1, which has a threshold voltage of −3 V, turns ON. At this time, the odd-numbered transfer thyristors T numbered 3 or higher, which have their cathode terminals connected to the transfer signal line 72-1, are unable to turn ON because their threshold voltage is −4.8 V. Meanwhile, the even-numbered transfer thyristors T are unable to turn ON because the transfer signal φ2 is “H” (0 V) and the transfer signal line 72-2 is “H” (0 V).

As the transfer thyristor T1 turns ON, the potential of the transfer signal line 72-1 becomes close to −1.5 V, which is a value equal to the potential at the anode (“H” (0 V)) minus the forward potential Vd (1.5 V) of the p-n junction.

When the transfer thyristor T1 turns ON, the potential of the gate Gt1/Gs1 becomes “H” (0 V), which is the potential at the anode of the transfer thyristor T1. Then, the potential of the gate Gt2 (the gate Gs2) becomes −1.5 V, the potential of the gate Gt3 (the gate Gs3) becomes −3 V, and the potential of the gates Gt (the gates Gs) that are numbered 4 or higher becomes “L”.

As a result, the threshold voltage of the setting thyristor S1 becomes −1.5 V, the threshold voltage of the transfer thyristor T2 and the setting thyristor S2 becomes −3 V, the threshold voltage of the transfer thyristor T3 and the setting thyristor S3 becomes −4.5 V, and the threshold voltage of the transfer thyristor T and the setting thyristor S that are numbered 4 or higher becomes −4.8 V.

However, since the transfer signal line 72-1 has become −1.5 V due to the transfer thyristor T1 that is in the ON state, the odd-numbered transfer thyristors T that are in their OFF state do not turn ON. Since the transfer signal line 72-2 is “H” (0 V), the even-numbered transfer thyristors T do not turn ON. Since the lighting signal line 75 is “H” (0 V), none of the laser diodes LD lights up.

Immediately after time b (which refers in this case to when a steady state is reached following a change in thyristors or other components occurring as a result of a change in signal potential at time b), the transfer thyristor T1 is in the ON state, and the transfer thyristors T other than the transfer thyristor T1, the setting thyristors S, and the laser diodes LD are in their OFF state.

Time c

At time c, the lighting signal φI shifts from “H” (0 V) to “Lo” (−5 V).

When the lighting signal φI shifts from “H” to “Lo”, the lighting signal line 75 shifts from “H” (0 V) to “Lo” (−5 V) via the current-limiting resistor RI and the φI terminal. Then, a voltage of −3.3 V, which equals −5 V plus 1.7 V applied to the laser diode LD, is applied to the setting thyristor S1, and the setting thyristor S1 having a threshold voltage of −1.5 V turns ON. The laser diode LD1 thus lights up (emits light). This places the lighting signal line 75 at a potential close to −3.2 V (a negative potential larger than 3.2 V in absolute value). At this time, although the threshold voltage of the setting thyristor S2 is set at −3 V, the voltage applied to the setting thyristor S2 is −1.5 V, which equals −3.2 V plus a voltage of 1.7 V applied to the laser diode LD. Thus, the setting thyristor S2 does not turn ON.

Immediately after time c, the transfer thyristor T1 and the setting thyristor S1 are is in the ON state, and the laser diode LD1 is lit (emitting light).

Time d

At time d, the lighting signal φI is shifted from “Lo” (−5 V) to “H” (0 V).

When the lighting signal φI shifts from “Lo” to “H”, the potential of the lighting signal line 75 shifts from −3.2 V to “H” (0 V) via the current-limiting resistor RI and the φI terminal. Then, the cathode of the laser diode LD1 and the anode of the setting thyristor S1 both become “H”. Thus, the setting thyristor S1 turns OFF, and the laser diode LD1 becomes extinguished (unlit). The lighting period of the laser diode LD1 represents the period in which a lighting signal φI1 is “Lo” (−5 V). This period extends from time c at which the lighting signal φI1 shifts from “H” to “Lo”, to time d at which the lighting signal φI1 shifts from “Lo” to Immediately after time d, the transfer thyristor T1 is in its ON state.

Time e

At time e, the transfer signal φ2 shifts from “H” (0 V) to “L” (−3.3 V). At this point, the period U(1) for performing lighting-control of the laser diode LD1 ends, and the period U(2) for performing lighting-control of the laser diode LD2 begins.

When the transfer signal φ2 shifts from “H” to “L”, the potential of the transfer signal line 72-2 shifts from “H” to “L” via the φ2 terminal. The transfer thyristor T2 has a threshold voltage set to −3 V as described above, and thus turns ON. As a result, the potential of the gate terminal Gt2 (the gate terminal Gs2) becomes “H” (0 V), the potential of the gate Gt3 (the gate Gs3) becomes −1.5 V, and the potential of the gate Gt4 (the gate Gs4) becomes −3 V. Then, the potential of the gate Gt (the gate Gs) numbered 5 or higher becomes −3.3 V.

Immediately after time e, the transfer thyristors T1 and T2 are in their ON state.

Time f

At time f, the transfer signal φ1 shifts from “L” (−3.3 V) to “H” (0 V).

When the transfer signal φ1 shifts from “L” to “H”, the potential of the transfer signal line 72-1 shifts from “L” to “H” via the φ1 terminal. Then, the anode and cathode of the transfer thyristor T1 that is in its ON state both become “H”, causing the transfer thyristor T1 to turn OFF. Then, the potential of the gate Gt1 (the gate Gs1) changes toward the power supply potential Vgk (“L” (−3.3 V)) of the power supply line 71 via the power line resistor Rg1. This brings the coupling diode D1 into a state (reverse biased state) in which a potential is applied to the coupling diode D1 in a direction that does not allow current flow. Therefore, the effect of the gate Gt2 (the gate Gs2) being “H” (0 V) does not extend to the gate Gt1 (the gate Gs1). That is, the threshold voltage of the transfer thyristor T having the gate Gt connected by the coupling diode D that is in reverse bias becomes −4.8 V, and thus such a thyristor T is not turned ON by the transfer signal φ1 or φ2 that is at “L” (−3.3 V).

Immediately after time f, the transfer thyristor T2 is in its ON state.

Others

At time g, as the lighting signal φI shifts from “H” (0 V) to “Lo” (−5 V), in the same manner as with the setting thyristor S1 and the laser diode LD1 at time c, the setting thyristor S1 turns ON, and the laser diode LD2 lights up (emits light).

At time h, as the lighting signal φI shifts from “Lo” (−5 V) to “H” (0 V), in the same manner as with the setting thyristor S1 and the laser diode LD1 at time d, the setting thyristor S2 turns OFF, and the laser diode LD2 is extinguished.

Further, at time i, as the transfer signal φ1 shifts from “H” (0 V) to “L” (−3.3 V), in the same manner as with the transfer thyristor T1 at time b or the transfer thyristor T2 at time e, the transfer thyristor T3 having a threshold voltage of −3 V turns ON. At time i, the period U(2) for performing lighting-control of the laser diode LD2 ends, and the period U(3) for performing lighting-control of the laser diode LD3 begins.

The same as described above applies to the subsequent operations.

As described above, the gate terminals Gt of the transfer thyristors T are connected to each other via the corresponding coupling diodes D. Thus, a change in the potential of a given gate Gt causes a change in the potential of the gate Gt connected via the coupling diode D that is in forward biased condition to the gate Gt whose potential has changed. This in turn causes a change in the threshold voltage of the transfer thyristor T that has the gate whose potential has changed. When the threshold voltage of the transfer thyristor T has a value higher than (a negative value smaller in absolute value than) “L” (−3.3 V), the transfer thyristor T turns ON at the timing when the transfer signal φ1 or the transfer signal φ2 shifts from “H” (0 V) to “L” (−3.3 V).

At this time, since the threshold voltage is −1.5 V for the setting thyristor S whose gate Gs is connected to the gate Gt of the transfer thyristor T that is in the ON state, the setting thyristor S turns ON when the lighting signal φI shifts from “H” (0 V) to “Lo” (−5 V), causing the laser diode LD connected in series with the setting thyristor S to light up (emit light).

That is, as the transfer thyristor T is placed in the ON state, the laser diode LD to be lighting-controlled is specified (selected), and the setting thyristor S is set to a state that enables the laser diode LD to light up. Then, the lighting signal φI at the “Lo” (−5 V) level causes the setting thyristor S to turn ON, and also causes the laser diode LD connected in series with the setting thyristor S to light up.

That is, the transfer signals φ1 and φ2 are used to sequentially specify the laser diodes LD, and the lighting signal φI is used to sequentially light the laser diodes LD that emit different wavelengths λ. As a result, the wavelength of light to be output is scanned (swept) from longer to shorter wavelengths, for example.

In an alternative configuration, instead of scanning (sweeping) the wavelength, the laser diode LD that outputs a desired wavelength may be selected by using the transfer signals φ1 and φ2, and upon selection of the corresponding laser diode LD, the lighting signal φI at the “Lo” (−5 V) level may be supplied to cause the laser diode LD to light up. At this time, if the laser diode LD that emits a wavelength other than a desired wavelength is being selected by the transfer signals φ1 and φ2, the lighting signal φI may be maintained at the “H” (0 V) level. Since the lighting signal φI at the “H” (0 V) level retains the setting thyristor S in its OFF state, the corresponding laser diode LD does not light up (emit light), either. The lighting signal φI sets lighting/unlighting of the laser diode LD.

Suppose that, after the laser diode LD that outputs a desired wavelength is selected by the transfer signals φ1 and φ2, it is desired to make that laser diode LD keep emitting light. This may be accomplished by maintaining the states of the transfer signals φ1 and φ2 and the lighting signal φI at that time.

Method for Fabricating Laser Component C

A method for fabricating the laser component C will be described. In the following, the method will be described with reference to the cross-section of the island 301 illustrated in FIG. 5A that is formed by stacking the setting thyristor S and the laser diode LD.

FIGS. 10A to 10F illustrate a method for fabricating the laser component C. FIG. 10A illustrates a semiconductor laminate forming step, FIG. 10B illustrates a step of forming the n-ohmic electrode 321, FIG. 10C illustrates an etching step for exposing the tunnel junction layer 85, FIG. 10D illustrates a step of forming the current-blocking portion β of the current confinement layer 86 b, FIG. 10E illustrates an etching step for exposing the p-gate layer 83, and FIG. 10F illustrates a step of forming the p-ohmic electrode 331 and the back electrode 91.

In FIGS. 10A to 10F, multiple steps are collectively illustrated in some cases.

These steps will be described below in order.

The semiconductor laminate forming step illustrated in FIG. 10A involves epitaxially growing, over the substrate 80 that is a p-type substrate, the p-anode layer 81, the n-gate layer 82, the p-gate layer 83, the n-cathode layer 84, the tunnel junction layer 85, the p-anode layer 86, the light emitting layer 87, and the n-cathode layer 88 in the stated order to form a semiconductor laminate. FIGS. 10A to 10F only depict the conductivity types of these layers, such as “p” and “n”.

Although the substrate 80 is made of a p-type GaAs material in this example, the substrate 80 may be made of an n-type GaAs material, or an intrinsic (i-type) GaAs material with no impurity doped. Alternatively, the substrate 80 may be a semiconductor substrate made of InP, GaN, InAs, or other group III-V or group II-VI materials, a semiconductor substrate made of a group IV material such as Si or Ge, or a substrate made of sapphire or other materials. If the substrate used is changed, a material that substantially matches (including a strain structure, a strain relaxation layer, and metamorphic growth) the lattice constant of the substrate is used as the material to be monolithically stacked over the substrate. In one example, a material such as InAs, InAsSb, or GaInAsSb is used as a material stacked over an InAs substrate, a material such as InP or InGaAsP is used as a material stacked over an InP substrate, a material such as GaN, AlGaN, or InGaN is used as a material stacked over a GaN substrate or a sapphire substrate, and a material such as Si, SiGe, or GaP is used as a material stacked over a Si substrate. For cases where the process involves crystal growth followed by bonding to another support substrate, the semiconductor material used does not need to be substantially lattice-matched to the support substrate. Further, the above step may be applied to the fabrication of a light emitting component using not only a semiconductor material but also an organic material having a p- or n-type conductivity similarly to a semiconductor material.

The p-anode layer 81 is a p-type Al_(0.9)GaAs layer with an impurity concentration of 1×10¹⁸/cm³, for example. The Al composition may be varied in the range from 0 to 1. The p-anode layer 81 may be made of GaInP or other materials.

The n-gate layer 82 is an n-type Al_(0.9)GaAs layer with an impurity concentration of 1×10¹⁷/cm³, for example. The Al composition may be varied in the range from 0 to 1. The n-gate layer 82 may be made of GaInP or other materials.

The p-anode layer 83 is a p-type Al_(0.9)GaAs layer with an impurity concentration of 1×10¹⁷/cm³, for example. The Al composition may be varied in the range from 0 to 1. The p-gate layer 83 may be made of GaInP or other materials.

The n-cathode layer 84 is an n-type Al_(0.9)GaAs layer with an impurity concentration of 1×10¹⁸/cm³, for example. The Al composition may be varied in the range from 0 to 1. The n-cathode layer 84 may be made of GaInP or other materials.

The tunnel junction layer 85 is formed by a junction (see FIG. 10B) of the n⁺⁺ layer 85 a doped with a high concentration of n-type impurities and the p⁺⁺ layer 85 b doped with a high concentration of p-type impurities. The n⁺⁺ layer 85 a and the p⁺⁺ layer 85 b each have a high impurity concentration of, for example, 1×10²⁰/cm³. The impurity concentration of a typical junction is on the order of 10¹⁷/cm³ to 10¹⁸/cm³. Examples of combinations of the n⁺⁺ layer 85 a and the p⁺⁺ layer 85 b (to be referred to as n⁺⁺ layer 85 a/p⁺⁺ layer 85 b hereinafter) include n⁺⁺GaInP/p⁺⁺GaAs, n⁺⁺GaInP/p⁺⁺AlGaAs, n⁺⁺GaAs/p⁺⁺GaAs, n⁺⁺AlGaAs/p⁺⁺AlGaAs, n⁺⁺InGaAs/p⁺⁺InGaAs, n⁺⁺GaInAsP/p⁺⁺GaInAsP, and n⁺⁺GaAsSb/p⁺⁺GaAsSb. These combinations may be changed with each other.

The p-anode layer 86 is formed by stacking the lower p-anode layer 86 a, the current confinement layer 86 b, and the upper p-anode layer 86 c in this order (see FIG. 10C).

The lower p-anode layer 86 a and the upper p-anode layer 86 c are, for example, p-type Al_(0.9)GaAs layers with an impurity concentration of 1×10¹⁸/cm³. The Al composition may be varied in the range from 0 to 1. These layers may be made of GaInP or other materials.

The current confinement layer 86 b is made of, for example, a p-type AlGaAs material with a high concentration of impurities such as AlAs or Al. The material used may be any material that, as Al is oxidized to form Al₂O₃, provides increased electrical resistance to confine the current path.

The light emitting layer 87 has a quantum well structure formed by an alternate stack of well and barrier layers. The well layer is made of, for example, GaAs, AlGaAs, InGaAs, GaAsP, AlGaInP, GaInAsP, or GaInP, and the barrier layer is made of, for example, AlGaAs, GaAs, GaInP, or GaInAsP. The light emitting layer 87 may be made of quantum lines (quantum wires) or quantum boxes (quantum dots).

The light emitting layer 87 may be formed by a combination of a well layer, a barrier layer, and spacer layers disposed above and below these layers. The light emitting layer 87 may be formed by, for example, a resonator structure.

The n-cathode layer 88 is, for example, an n-type Al_(0.9)GaAs layer with an impurity concentration of 1×10¹⁸/cm³. The Al composition may be varied in the range from 0 to 1. The n-cathode layer 88 may be made of GaInP or other materials.

These semiconductor layers are stacked by, for example, metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) to form a semiconductor laminate.

In the step of forming the n-ohmic electrode 321 illustrated in FIG. 10B, first, the n-ohmic electrode 321 is formed over the n-cathode layer 88.

The n-ohmic electrode 321 is made of, for example, Au (AuGe) containing Ge that allows for easy ohmic contact to an n-type semiconductor layer such as the n-cathode layer 88.

The n-ohmic electrode 321 is formed by, for example, a lift-off method.

In the etching step for exposing the tunnel junction layer 85 illustrated in the FIG. 10C, the n-cathode layer 88, the light emitting layer 87, and the p-anode layer 86 over the tunnel junction layer 85 are etched away around the laser diode LD.

This etching may be performed by wet etching using a sulfuric acid-based etchant (with a weight ratio between sulfuric acid, aqueous hydrogen peroxide, and water=1:10:300) or other etchants, or by anisotropic drying etching (e.g., RIE) using, for example, boron chloride.

In the step of forming the current-blocking portion β of the current confinement layer 86 b illustrated in FIG. 10D, the current confinement layer 86 b, which has its side surface exposed by the etching step performed for exposing the tunnel junction layer 85, is oxidized from the side surface to form the current-blocking portion β that blocks current. The portion of the current confinement layer 86 b left unoxidized serves as the current-passing portion α.

The oxidation of the current confinement layer 86 b from its side surface is performed by, for example, oxidizing Al of the current confinement layer 86 b made of AlAs, AlGaAs, or other materials by steam oxidation at a temperature of 300° C. to 400° C. At this time, oxidation proceeds from the exposed side surface, and the current-blocking portion β made of Al₂O₃, which is an oxide of Al, is formed around the laser diode LD.

The current-blocking portion β may be formed by, instead of oxidation of AlAs, implantation of hydrogen (H₂). That is, the lower p-anode layer 86 a and the upper p-anode layer 86 c may be deposited not separately but contiguously as the p-anode layer 86, followed by implantation of hydrogen (H₂) ions into the portion of the p-anode layer 86 that is to become the current-blocking portion β. This imparts insulating properties to materials such as Al_(0.9)GaAs to form the current-blocking portion β.

In the etching step for exposing the p-gate layer 83 illustrated in FIG. 10E, the tunnel junction layer 85 and the n-cathode layer 84 are etched to expose the p-gate layer 83.

This etching may be performed by wet etching using a sulfuric acid-based etchant (with a weight ratio between sulfuric acid, aqueous hydrogen peroxide, and water=1:10:300), or by anisotropic drying etching using, for example, boron chloride.

If, in the etching step illustrated in FIG. 10C performed for exposing the tunnel junction layer 85, the p-gate layer 83 is exposed instead of the tunnel junction layer 85, this may cause Al included in the p-gate layer 83 to be oxidized in the step of forming the current-blocking portion β illustrated in FIG. 10D. Such oxidation of Al included in the p-gate layer 83 causes, for example, surface roughness, or poor adhesion of the p-ohmic electrode 331 described later. Accordingly, the step of forming the current-blocking portion β is performed with the tunnel junction layer 85 exposed.

In the step of forming the p-ohmic electrode 331 and the back electrode 91 illustrated in FIG. 10F, first, the p-ohmic electrode 331 is formed over the p-gate layer 83.

The p-ohmic electrode 331 is made of, for example, Au (AuZn) containing Zn that allows for easy ohmic contact to a p-type semiconductor layer such as the p-gate layer 83.

The p-ohmic electrode 331 is formed by, for example, a lift-off method. At this time, the other p-ohmic electrodes may be formed simultaneously.

Next, the back electrode 91 is formed on the back of the substrate 80.

As with the p-ohmic electrode 331, the back electrode 91 is made of, for example, AuZn.

In addition to the steps mentioned above, the fabrication process includes steps such as forming the protective layer 90, forming through-holes in the protective layer 90, forming wiring lines (the connection wiring lines 76 and 77), and forming, by means of cleavage, an emitting surface from which a laser beam is emitted.

The method for fabricating the laser component C has been described above with respect to a portion of the island 301 formed by stacking the setting thyristor S and the laser diode LD.

The other islands including the transfer thyristors T, the coupling diodes D, the power line resistor Rg1, and current-limiting resistors R1 to R6, such as the islands 302 to 306, are formed through a process including, in addition to the above-mentioned steps, the steps of exposing the surface of the n-cathode layer 84, and forming components such as an n-ohmic electrode 322 or 323 over the n-cathode layer 84 that has been exposed. The step of forming wiring lines includes forming lines such as the power supply line 71, the transfer signal lines 72-1 and 72-2, and the lighting signal line 75.

In the above-mentioned example, the p-ohmic electrode 331 is formed over the p-gate layer 83 to serve as the gate terminal Gs of the setting thyristor S. Alternatively, the gate terminal Gs of the setting thyristor S may be disposed over the n-gate layer 82. The same applies to the transfer thyristor T.

Instead of forming the current confinement layer 86 b in the p-anode layer 86, the current confinement layer 86 b may be formed in the p-anode layer 81.

In the first exemplary embodiment, the laser diode LD and the setting thyristor S are stacked with the tunnel junction layer 85 interposed therebetween. In this case, the tunnel junction layer 85 of the laser diode LD becomes reverse biased. In this regard, a tunnel junction has a characteristic of allowing current to flow even in reversed biased condition.

If the tunnel junction layer 85 is not provided, the junction between the laser diode LD and the setting thyristor S is reverse biased. Accordingly, to allow current to flow through the laser diode LD and the setting thyristor S, a voltage that causes the reverse biased junction to break down needs to be applied. This translates into increased driving voltage.

Therefore, stacking the laser diode LD and the setting thyristor S with the tunnel junction layer 85 interposed therebetween helps to keep the driving voltage lower than that required if the tunnel junction layer 85 is not provided between the laser diode LD and the setting thyristor S.

As described above, the tunnel junction layer 85 allows current to flow relatively easily in reverse biased condition. However, the junction between the n-cathode layer 84 and the p-anode layer 86, which is not a tunnel junction, does not readily allow current to flow under a reverse bias that does not cause the junction to break down. Accordingly, the tunnel junction layer 85 may be formed in the portion corresponding to the current-passing portion α, and the tunnel junction layer 85 may not be formed in the portion corresponding to the current-blocking portion β. In this case, after deposition of the tunnel junction layer 85, a part of the tunnel junction layer 85 is etched away, and then the p-anode layer 86 is epitaxially grown so as to bury the tunnel junction layer 85 that is left to remain. The area around the tunnel junction layer 85 that is left to remain may be buried by the n-cathode layer 84 instead of the p-anode layer 86. This configuration may be employed for applications that use semiconductor materials for which it is difficult to use steam oxidation.

The following describes a modification of the laser component C according to the first exemplary embodiment. The modification described below uses, instead of the tunnel junction layer 85, a group III-V compound layer that exhibits metal-like conductivity and epitaxially grows into a group III-V compound semiconductor layer. In this case, the “tunnel junction layer 85” in the foregoing description of the first exemplary embodiment may be read as “metal-like-conductivity group III-V compound layer 85” described below.

FIGS. 11A to 11C illustrate materials forming the metal-like-conductivity group III-V compound layer. FIG. 11A illustrates the band gap of InNAs with respect to the composition ratio x of InN, FIG. 11B illustrates the band gap of InNSb with respect to the composition ratio x of InN, and FIG. 11C illustrates the lattice constants of group VI elements and group III-V compounds with respect to band gap.

FIG. 11A illustrates band gap energy (eV) with respect to InNAs, which is a compound of InN having a composition ratio x (x=0 to 1) and InAs having a composition ratio (1−x).

FIG. 11B illustrates band gap energy (eV) with respect to InNSb, which is a compound of InN having a composition ratio x (x=0 to 1) and InSb having a composition ratio (1−x).

InNAs and InNSb, which are each herein described as an example of a material forming the metal-like-conductivity group III-V compound layer, are known to have a negative band gap energy in a certain range of composition ratios x as illustrated in FIGS. 11A and 11B. Having a negative band gap energy means having no band gap. Hence, such a material exhibits conductivity characteristics (conduction characteristics) similar to those of metals. In other words, metal-like conductive characteristics (conductivity) mean that as with metals, current is allowed to flow in the presence of a potential gradient.

As illustrated in FIG. 11A, InNAs has a negative band gap energy when, for example, the composition ratio x of InN is in the range of approximately 0.1 to approximately 0.8.

As illustrated in FIG. 11B, InNSb has a negative band gap energy when, for example, the composition ratio x of InN is in the range of approximately 0.2 to approximately 0.75.

That is, InNAs and InNSb exhibit metal-like conductive characteristics (conductivity) in the above-mentioned ranges.

In regions with small band gap energies outside the above-mentioned ranges, the electrons have energy due to thermal energy, allowing the electrons to transition across a small band gap. Thus, these materials exhibit characteristics that readily allow current to flow in the presence of a potential gradient, as with when the band gap energy is negative or as with metals.

Even if InNAs and InNSb contain elements such as Al, Ga, Ag, and P, depending on the composition, the band gap energy of these materials can be maintained to be near zero or negative, which allows current to flow in the presence of a potential gradient.

Further, as illustrated in FIG. 11C, group III-V compounds (semiconductors) such as GaAs and InP have lattice constants ranging from 5.6 Å to 5.9 Å. These lattice constants are close to the lattice constant of Si, which is approximately 5.43 Å, and to the lattice constant of Ge, which is approximately 5.66 Å.

In contrast, InN, which is likewise a group III-V compound, has a lattice constant of approximately 5.0 Å in the case of the zinc blend structure, and InAs has a lattice constant of approximately 6.06 Å. Hence, the lattice constant of InNAs, which is a compound of InN and InAs, can take values close to 5.6 Å to 5.9 Å, which coincide with the range of lattice constants of materials such as GaAs.

InSb, which is a group III-V compound, has a lattice constant of approximately 6.48 Å. Since the lattice constant of InN is approximately 5.0 Å, the lattice constant of InNSb, which is a compound of InSb and InN, can take values close to 5.6 Å to 5.9 Å, which coincide with the range of lattice constants of materials such as GaAs.

That is, InNAs and InNSb can be epitaxially grown monolithically over a layer of a group III-V compound (semiconductor) such as GaAs. Further, a layer of a group III-V compound (semiconductor) such as GaAs can be epitaxially grown monolithically over a layer of InNAs or InNSb.

Therefore, if the laser diode LD and the setting thyristor S are stacked to be connected in series with each other such that, instead of the tunnel junction layer 85, a metal-like-conductivity group III-V compound layer is interposed between the laser diode LD and the setting thyristor S, this prevents or reduces reverse biasing of the junction between the p-anode layer 86 of the laser diode LD and the n-cathode layer 84 of the setting thyristor S.

A metal-like-conductivity group III-V compound layer readily allows current to flow. However, the junction between the n-cathode layer 84 and the p-anode layer 86 does not readily allow current to flow under a reverse bias that does not cause the junction to break down. Accordingly, the metal-like-conductivity group III-V compound layer may be formed in the portion corresponding to the current-passing portion α, and the metal-like-conductivity group III-V compound layer may not be formed in the portion corresponding to the current-blocking portion β.

As described above, the laser component C according to the first exemplary embodiment is formed by stacking the setting thyristor S and the laser diode LD. Thus, the laser component C constitutes a self-scanning light emitting device (SLED) array in which the laser diodes LD are sequentially lit up by means of the transfer thyristor T and the setting thyristor S. This configuration reduces the number of terminals provided in the laser component C, leading to miniaturization of the laser component C and the laser generating unit 10.

In some cases, the laser diode LD is not disposed over the setting thyristor S, and the setting thyristor S is used as a laser device. That is, in some cases, the p-anode layer 81 and the n-cathode layer 84 of the setting thyristor S serve as cladding layers, and the light emitted at the junction of the n-gate layer 82 and the p-gate layer 83 in the ON state is used as a laser beam. In such cases, it is not possible to set the transfer characteristics due to the transfer thyristor T and the setting thyristor S, and the light emission characteristics of the laser device (the setting thyristor S) individually (independently). This makes it difficult to achieve improvements such as faster driving, higher optical output, higher efficiency, lower power consumption, and lower cost.

Further, suppose that, for example, a thyristor (the setting thyristor S) is used as a laser device to emit light at 780 nm. In this case, to construct a quantum well structure using AlGaAs, the composition ratio of Al is set to 30%. In this case, if etching for exposing the gate is performed, Al is oxidized, making it impossible to form the gate terminal.

In contrast, in the first exemplary embodiment, light emission is performed by the laser diode LD, and transfer is performed by the transfer thyristor T and the setting thyristor S. Thus, light emission and transfer are separated from each other. The setting thyristor S is not required to emit light. This allows the laser diode LD to be constructed as a quantum well structure to improve characteristics such as light emission characteristics, and also makes it possible to improve characteristics such as the transfer characteristics due to the transfer thyristor T and the setting thyristor S. That is, this configuration allows the laser diode LD of the light emitting unit 102, and the transfer thyristor T and the setting thyristor S of the driving unit 101 to be set individually (independently). This makes it easy to achieve improvements such as faster driving, higher optical output, higher efficiency, lower power consumption, and lower cost.

FIG. 12 illustrates the laser generating unit 10 including multiple laser components C. The laser generating unit 10 includes the following laser components C arranged in parallel in the same plane: the laser component C that emits light at wavelengths λ1 to λ1000, the laser component C that emits light at wavelengths λ1001 to λ2000, and the laser components C that emit light at wavelengths different from the wavelengths mentioned above. The signal generating circuit 110 is provided for each laser component C. Using multiple laser components C that differ in emission wavelength as described above allows for increased bandwidth.

Since the laser components C are each provided with the signal generating circuit 110 as described above, the laser components C are made to operate in parallel. Such parallel operation enables faster operation of the laser generating unit 10.

Second Exemplary Embodiment

With the laser component C of the laser generating unit 10 according to the first exemplary embodiment, the laser diodes LD are sequentially selected by using the transfer thyristors T. With the laser component C of the laser generating unit 10 according to a second exemplary embodiment, the setting thyristors S are used as transfer thyristors. The configuration of the second exemplary embodiment is otherwise the same as that of the first exemplary embodiment, and thus differences from the first exemplary embodiment will be described below.

With the laser component C according to the first exemplary embodiment, as described above with reference to FIG. 3, the gate Gs of the setting thyristor S is connected to the gate Gt of the transfer thyristor T. This means that it is possible to substitute the setting thyristor S for the transfer thyristor T, and cause the setting thyristor S to act as the transfer thyristor T.

FIG. 13 is an equivalent circuit diagram of the laser component C according to the second exemplary embodiment, and illustrates the signal generating circuit 110 that supplies signals such as signals used to drive the laser component C.

The signal generating circuit 110 does not include the lighting signal generating unit 140.

The configuration of the laser component C is such that the transfer thyristors T illustrated in FIG. 3 are removed, and the setting thyristors S serve as the transfer thyristors T. Otherwise, the configuration according to the second exemplary embodiment is the same as that of the laser component C according to the first exemplary embodiment illustrated in FIG. 3.

In this case, the “L” level of the transfer signals φ1 and φ2 transmitted by the transfer signal generating unit 120 of the signal generating circuit 110 of the laser generating unit 10 according to the first exemplary embodiment may be changed to “Lo”. “Lo” refers to a voltage that causes the setting thyristor S of the laser generating unit 10 according to the first exemplary embodiment to turn ON, and also causes the laser diode LD to light up. However, this causes the laser diodes LD1 and LD2 to light up simultaneously when, in the period from time e to time f illustrated in FIG. 9, the transfer thyristors T1 and T2 are both in their ON state, for example.

Accordingly, a period of “Lo′” level may be provided prior to transition to the “Lo” level. The “Lo′” level represents a level that causes the setting thyristor S to turn ON but causes the laser diode LD to light up in low light quantity condition.

FIG. 14 is a timing chart illustrating the operation of the laser generating unit 10 according to the second exemplary embodiment. Features that are similar to those in FIG. 9 described above with reference to the first exemplary embodiment will be denoted by the same reference signs to avoid repetitive descriptions, and differences from the first exemplary embodiment will be described.

The lighting signal φI illustrated in FIG. 9 is not provided. Instead, the transfer signals φ1 and φ2 are changed from those in the first exemplary embodiment. The transfer signal generating unit 120 of the signal generating circuit 110 causes the transfer signal φ1 to shift from “H” (0 V) to “Lo′” at time b. At this time, the laser diode LD lights up in low light quantity condition. Then, at time c, the transfer signal φ1 is shifted from “Lo′” to “Lo” (5 V). This causes the laser diode LD1 to light up with a predetermined light quantity.

Then, the transfer signal φ1 is shifted from “Lo” (5 V) to “Lo′” at time d, and shifted from “Lo′” to “H” (0 V) at time f.

The transfer signal φ2 is a signal shifted backward by the period U relative to the transfer signal φ1.

In this way, the transfer thyristor T is made to double as the transfer thyristor T and the setting thyristor S illustrated in FIG. 3.

As described above, with the laser generating unit 10 according to the second exemplary embodiment, the laser diode LD of the laser component C lights up in low light quantity condition when the transfer signals φ1 and φ2 are at the “Lo′” level. This light quantity may be set to a value that does not affect the overall operation.

The fact that the transfer thyristor T doubles as the setting thyristor S, and the absence of the lighting signal line 75 allow for miniaturization of the laser component C.

For the laser component C, instead of using the laser diode LD, the transfer thyristor T may be constructed as a laser thyristor if this does not adversely affect transfer and light emission characteristics. For example, laser oscillation may be produced with the p-anode layer 81 and the n-cathode layer 84 used as cladding layers. This configuration eliminates the need to stack the tunnel junction layer 85, the p-anode layer 86, the light emitting layer 87, and the n-cathode layer 88 in the case of the arrangement illustrated in FIG. 10A. This facilitates fabrication of the laser component C.

Third Exemplary Embodiment

The optical intensity of the laser diode LD is subject to fluctuations (variations) caused by factors such as an oscillation delay that occurs upon application of voltage, or relaxation oscillation that occurs after the laser diode LD starts to oscillate. The term optical intensity as used herein refers to radiant intensity (or luminosity).

FIG. 15 illustrates changes in the optical intensity P of the laser diode LD with time. The vertical axis represents optical intensity P, and the horizontal axis represents time t.

Suppose that voltage is applied to the laser diode LD at the “On” timing along time t, and the application of voltage to the laser diode LD is stopped at the “Off” timing. At this time, an ideal response waveform Ri is such that a predetermined optical intensity P is maintained from the “On” timing to the “Off” timing.

In actuality, however, there is an oscillation delay time td between the “On” timing and the start of oscillation of the laser diode LD. Even after oscillation begins, relaxation oscillation that causes fluctuations in optical intensity P occurs (see a relaxation oscillation waveform Rr). The relaxation oscillation continues for a relaxation oscillation duration tr.

Thus, factors such as an oscillation delay or relaxation oscillation occurring at the “On” timing tend to give rise to, for example, fluctuations (variations) in optical intensity P or fluctuations (variations) in the amount of light (radiant energy) obtained in the period of time from the “On” timing to the “Off” timing. Consequently, such oscillation delay, relaxation oscillation, or other factors make it difficult to shorten the interval of time between the “On” timing and the “Off” timing. In other words, it is difficult to perform high speed optical switching.

For example, the sum of the oscillation delay time td and the relaxation oscillation duration tr equals approximately five nsec. Therefore, if the laser devices LD are to be simply switched from the OFF state to the ON state sequentially, this makes it impossible to switch the laser diodes LD at speeds equal to or higher than approximately 200 Mbps. In this regard, for high speed operation, speeds of 1 Gbps or over is generally required.

Once the relaxation oscillation duration tr elapses, however, the fluctuations in optical intensity P represented by the relaxation oscillation waveform Rr subside. Thereafter, the optical intensity P is set by the current passed through the laser diode LD.

Accordingly, in the third exemplary embodiment, the lighting signal φI is supplied in two steps. That is, with the optical intensity P represented by a level “0/1”, before the time at which a transition to the optical intensity P corresponding to a level “1” (ON state of level “1”) occurs, a period of low optical intensity P regarded as (corresponding to) level “0” (ON state of level “0”) is provided. The optical intensity P at level “0” represents the optical intensity when the laser diode LD is in its OFF state. The optical intensity P at level “1” represents the optical intensity at which light is desired to be emitted. The optical intensity P regarded to be of level “0” corresponds to, for example, such an optical intensity that may not be detected by the photo-detector 4, as is the case with when the laser diode LD is in the OFF state.

Instead of being a binary value of “0/1”, the level of optical intensity may be multivalued including a level “m: m is an integer equal to or greater than 1” and a level “0”. In the following, assuming that level “1” includes level “m”, a description will be given using level “0” and level “1”. A level may be referred to as logical value.

FIGS. 16A and 16B illustrate the optical intensity P of the laser diode LD. FIG. 16A illustrates optical intensity P with respect to current I, and FIG. 16B illustrates changes in optical intensity P with respect to time t.

As illustrated in FIG. 16A, the laser diode LD starts to oscillate once the current I exceeds a threshold current Ith. Accordingly, it is assumed that the following currents are supplied: a current I (“0”) equal to or higher than the threshold current Ith and at which the optical intensity P is regarded as level “0”, and a current I (“1”) corresponding to the optical intensity P at level “1”. It is assumed that the voltage applied to the laser diode LD to supply the current I (“0”) is V (“0”), and the voltage applied to the laser diode LD to supply the current I (“1”) is V (“1”). For example, V (“0”) is set to 1.5 V, and V (“1”) is set to 1.7 V.

As illustrated in FIG. 16B, first, the voltage applied to the laser diode LD is set to V (“0”), and the laser diode LD is made to oscillate (light up) in the ON state of level “0”. An oscillation delay or relaxation oscillation is allowed to occur in this state. Thereafter, the voltage applied to the laser diode LD is set to V (“1”), causing the laser diode LD to switch to the ON state of level “1”. Then, the voltage applied to the laser diode LD is set to 0 V (“H”) to turn OFF the laser diode LD.

When the voltage V (“1”) is applied to the laser diode LD on which V (“0”) has been applied, the laser diode LD immediately goes to the ON state of level “1”. During the period of level “1” (period t in FIG. 16B), the laser diode LD is not subject to the influence of an oscillation delay or relaxation oscillation. Such an oscillation delay or relaxation oscillation is absorbed in a period σ preceding the period τ and representing the ON state of level “0”.

Like the laser generating unit 10 according to the first exemplary embodiment, the laser generating unit 10 according to the third exemplary embodiment includes the laser component C including the laser diodes LD arranged as a light emitting device (laser device) array, and the signal generating circuit 110 that drives the laser component C. The configuration of the third exemplary embodiment is otherwise the same as that of the first exemplary embodiment, and thus differences from the first exemplary embodiment will be described below.

FIG. 17 is an equivalent circuit diagram of the laser component C, and illustrates the signal generating circuit 110 that supplies signals such as signals used to drive the laser component C. It is to be noted that the laser component C and the signal generating circuit 110 are complex in configuration in comparison to the laser generating unit 10 according to the first exemplary embodiment illustrated in FIG. 3.

Laser Generating Unit 10

First, the signal generating circuit 110 will be described. The signal generating circuit 110 includes transfer signal generating units 120 a and 120 b that generate signals for sequentially setting the transfer thyristors T to the ON state, the lighting signal generating unit 140 that generates signals for causing the laser diodes LD to light up, the reference-potential supply unit 160 that supplies the reference potential Vsub, and power-supply-potential supply units 170 a and 170 b that respectively supply power supply potentials Vgk1 and Vgk2 that are required for driving.

The transfer signal generating unit 120 a generates the transfer signals φ1 and φ2 and a start signal φs1, and the transfer signal generating unit 120 b generates transfer signals φ3 and φ4 and a start signal φs2. For the ease of illustration, FIG. 17 depicts the transfer signal generating unit 120 a and the transfer signal generating unit 120 b separately. The transfer signal generating units 120 a and 120 b will be sometimes referred to as transfer signal generating unit 120 when not distinguished from each other. The transfer signals φ1 to φ4 will be sometimes referred to as transfer signal φ when not distinguished from each other.

The lighting signal generating unit 140 transmits lighting signals φI1 and φI2 (to be referred to as lighting signal φI when not distinguished from each other).

The power-supply-potential supply unit 170 a supplies the power supply potential Vgk1, and the power-supply-potential supply unit 170 b supplies the power supply potential Vgk2. For the ease of illustration, FIG. 17 depicts the power-supply-potential supply unit 170 a and the power-supply-potential supply unit 170 b separately. The power-supply-potential supply units 170 a and 170 b will be sometimes referred to as power-supply-potential supply unit 170 when not distinguished from each other. The power supply potentials Vgk1 and Vgk2 will be sometimes referred to as Vgk when not distinguished from each other.

Next, the laser component C will be described. Like the laser component C according to the first exemplary embodiment, the laser component C includes the laser diodes LD1, LD2, LD3, and so on, the setting thyristor S1, S2, S3, and so on, the transfer thyristors T1 and T2, T3, and so on, the coupling diodes D1, D2, D3, and so on, and the power line resistors Rg1, Rg2, Rg3, and so on. FIG. 17 illustrates the portion of the laser component C related to the laser diodes LD1 to LD8.

With each two of the odd-numbered transfer thyristors T1, T3, T5, and so on paired in numerical order, each of the odd-numbered coupling diodes D1, D3, D5, and so on is located between each two paired transfer thyristors T. With each two of the even-numbered transfer thyristors T2, T4, T6, and so on paired in numerical order, each of the even-numbered coupling diodes D2, D4, D6, and so on is located between each two paired transfer thyristors T. The number of coupling diodes D is, for example, 126 assuming that the number of laser diodes LD is 128.

The laser component C includes the current-limiting resistors R1 to R4. The current-limiting resistors R1 to R4 are respectively disposed in transfer signal lines 72-1 to 72-4 (to be referred to as transfer signal line 72 when not distinguished from each other) on which the transfer signals φ1 to φ4 described later are transmitted, to prevent excessive current from flowing through the transfer signal lines 72-1 to 72-4. Further, the laser component C includes the current-limiting resistors R5 and R6. The current-limiting resistors R5 and R6 are respectively disposed in start signal lines 73-1 and 73-2 (to be referred to as start signal line 73 when not distinguished from each other) on which the start signal φs1 and φs2 are transmitted, to prevent excessive current from flowing through the start signal lines 73-1 and 73-2. The current-limiting resistors R1 to R6 will be sometime referred to as current-limiting resistor R.

In this case, components such as the transfer thyristors T1 and T2, T3, and so on, the setting thyristor S1, S2, S3, and so on, the power line resistor Rg1, Rg2, Rg3, and so on, the coupling diodes D1, D2, D3, and so on, the multiple current-limiting resistors R1 to R6, the transfer signal lines 72-1 to 72-4, the start signal lines 73-1 and 73-2, and lighting signal lines 75-1 and 75-2 that will be described later constitute the driving unit 101, and the light emitting device (laser device) array made up of the laser diodes LD1, LD2, LD3, and so on constitutes the light emitting unit 102.

Next, electrical connection of various elements in the laser component C will be described.

The respective anodes of the transfer thyristor T and the setting thyristor S are connected to the substrate 80 of the laser component C (anode-common configuration).

These anodes are supplied with the reference potential Vsub from the reference-potential supply unit 160 via the back electrode 91 (see FIG. 4B described above with reference to the first exemplary embodiment), which is a Vsub terminal disposed on the back of the substrate 80.

This connection represents an example of configuration when the substrate 80 is a p-type substrate. The polarity is reversed if an n-type substrate is used. If an intrinsic (i-type) substrate with no impurity doped is to be used, the terminal to be connected to the reference potential Vsub is disposed at the side of the substrate 80 where the driving unit 101 and the light emitting unit 102 are located.

The anode of the laser diode LD is connected to the cathode of the same-numbered setting thyristor S.

The gate Gs of the setting thyristor S is connected to the gate Gt of the same-numbered transfer thyristor T.

Let n (n is an integer equal to or greater than 1) be a number representing the transfer thyristor T among the odd-numbered transfer thyristors T1, T3, T5, T7, and so on along the array of the transfer thyristors T. In this case, the cathodes of the transfer thyristors T that are numbered 1+4×(n−1) (the transfer thyristors T1, T5, and so on in FIG. 17) are connected to the transfer signal line 72-1. The transfer signal line 72-1 is connected to the φ1 terminal via the current-limiting resistor R1. The transfer signal φ1 is transmitted to the φ1 terminal from the transfer signal generating unit 120 a.

Among the odd-numbered transfer thyristors T1, T3, T5, T7, and so on, the cathodes of the transfer thyristors T that are numbered 3+4×(n−1) (the transfer thyristors T3, T7, and so on in FIG. 17) are connected to the transfer signal line 72-2. The transfer signal line 72-2 is connected to the φ2 terminal via the current-limiting resistor R2. The transfer signal φ2 is transmitted to the φ2 terminal from the transfer signal generating unit 120 a.

The gates Gt of the transfer thyristors T that are numbered 1+4×(n−1) (the transfer thyristors T1, T5, and so on in FIG. 17) are connected to the anodes of the same-numbered coupling diodes D. The cathodes of these coupling diodes D are connected to the gates Gt of the transfer thyristors T that are numbered 3+4×(n−1) (the transfer thyristors T3, T7, and so on in FIG. 17).

The anode of the gate Gt1 is connected to the φs1 terminal via the current-limiting resistor R5. The start signal φs1 is transmitted to the φs1 terminal from the transfer signal generating unit 120 a.

Let n (n is an integer equal to or greater than 1) be a number representing the transfer thyristor T among the even-numbered transfer thyristors T2, T4, T6, T8, and so on along the array of the transfer thyristors T. In this case, the cathodes of the transfer thyristors T that are numbered 2+4×(n−1) (the transfer thyristors T2, T6, and so on in FIG. 17) are connected to the transfer signal line 72-3. The transfer signal line 72-3 is connected to a φ3 terminal via the current-limiting resistor R3. The transfer signal φ3 is transmitted to the φ3 terminal from the transfer signal generating unit 120 b.

Among the even-numbered transfer thyristors T2, T4, T6, T8, and so on, the cathodes of the transfer thyristors T that are numbered 4+4×(n−1) (the transfer thyristors T4, T8, and so on in FIG. 17) are connected to the transfer signal line 72-4. The transfer signal line 72-4 is connected to a φ4 terminal via the current-limiting resistor R4. The transfer signal φ4 is transmitted to the 04 terminal from the transfer signal generating unit 120 b.

The gates Gt of the transfer thyristors T that are numbered 2+4×(n−1) (the transfer thyristors T2, T6, and so on in FIG. 17) are connected to the anodes of the same-numbered coupling diodes D. The cathodes of these coupling diodes D are connected to the gates Gt of the transfer thyristors T that are numbered 4+4×(n−1) (the transfer thyristors T4, T8, and so on in FIG. 17).

The anode of the gate Gt2 is connected to the φs2 terminal via the current-limiting resistor R6. The start signal φs2 is transmitted to the φs2 terminal from the transfer signal generating unit 120 b.

The respective gates Gt1, Gt3, Gt5, Gt7, and so on of the odd-numbered transfer thyristors T1, T3, T5, T7, and so on are connected to the power supply line 71-1 via the same-numbered power line resistors Rg. The power supply line 71-1 is connected to the Vgk1 terminal. The power supply potential Vgk1 is supplied to the Vgk1 terminal from the power-supply-potential supply unit 170 a.

The respective gates Gt2, Gt4, Gt6, Gt8, and so on of the even-numbered transfer thyristors T2, T4, T6, T8, and so on are connected to the power supply line 71-2 via the same-numbered power line resistors Rg. The power supply line 71-2 is connected to the Vgk2 terminal. The power supply potential Vgk2 is supplied to the Vgk2 terminal from the power-supply-potential supply unit 170 b.

The cathodes of the odd-numbered laser diodes LD1, LD3, LD5, LD7, and so on are connected to a φI1 terminal via the lighting signal line 75-1. The lighting signal φI1 is transmitted to the φI1 terminal from the lighting signal generating unit 140 via a current-limiting resistor RI1 disposed outside the laser component C.

The cathodes of the even-numbered laser diodes LD2, LD4, LD6, LD8, and so on are connected to a φI2 terminal via the lighting signal line 75-2. The lighting signal φI2 is transmitted to the φI2 terminal from the lighting signal generating unit 140 via a current-limiting resistor RI2 disposed outside the laser component C.

The lighting signal lines 75-1 and 75-2 will be referred to as lighting signal line 75 when not distinguished from each other.

The current-limiting resistors RI1 and RI2 may be disposed inside the laser component C. The current-limiting resistors RI1 and RI2 will be sometimes referred to as current-limiting resistor RI.

As described above, with the laser component C according to the third exemplary embodiment, the set of odd-numbered laser diodes LD1, LD3, LD5, and so on (the set of odd-numbered laser diodes LD), and the set of even-numbered laser diodes LD2, LD4, LD6, and so on (the set of even-numbered laser diodes LD) are constructed independently and combined in the numerical order of the laser diodes LD.

The plan layout and cross-sectional structure of the laser component C according to the third exemplary embodiment may be those obtained by partially modifying the laser component C according to the first exemplary embodiment illustrated in FIGS. 4A and 4B. Accordingly, this will not be described further in detail.

Operation of Laser Generating Unit 10

Timing Chart

FIG. 18 is a timing chart illustrating the operation of the laser generating unit 10 according to the third exemplary embodiment.

The timing chart illustrated in FIG. 18 represents a control to light (to be referred to as lighting control) eight laser diodes, the laser diodes LD1 to LD8, of the laser component C in this order.

It is assumed that in FIG. 18, time passes in alphabetical order from time a to time w. The laser diode LD1 is lighting-controlled in the period U(1) (from time a to time f), the laser diode LD2 is lighting-controlled in the period U(2) (from time a to time k), and the laser diode LD3 is lighting-controlled in the period U(3) (from time f to time p). The laser diodes LD that are numbered 4 or higher are subsequently lighting-controlled in the same manner. The period U(1) represents the period for activating the laser component C, and differs from the other periods. It is assumed that the periods U(2), U(3), U(4), and so on are of the same length, and will be referred to as period U when not distinguished from each other.

For example, the period from time e to time k in the latter half of the period U(2), which is a period for performing lighting-control of the laser diode LD2, overlaps the first half of the period U(3), which is a period for performing lighting control of the laser diode LD3. That is, the period U for performing lighting-control of the set of odd-numbered laser diodes LD, and the period U for performing lighting-control of the set of even-numbered laser diodes LD are shifted relative to each other by ½ of the period U.

The transfer signals φ1 to φ4 respectively transmitted to the φ1 to φ4 terminals, and the start signals φs1 and φs2 respectively transmitted to the φ1s and φs2 terminals each have two levels of potentials, “H” (0 V) and “L” (−3.3 V).

The start signal φs1 causes transfer (propagation) of the ON state to be started for the odd-numbered transfer thyristors T. Likewise, the start signal φs2 causes transfer (propagation) of the ON state to be started for the even-numbered transfer thyristors T.

The start signal φs1 is “H” at time a, shifts from “H” to “L” at time g, and then maintains its “L” level.

The start signal φs2 is “L” at time a, and shifts from “L” to “H” at time b. Then, the start signal φs2 shifts from “H” to “L” at time l, and thereafter maintains its “L” level.

Each of the transfer signals φ1 to φ4 repeats its waveform every two consecutive periods U (e.g., the period U(3) and the period U(5), or the period U(2) and the period U(4)).

The transfer signal φ1 is “L” at time a, and shifts from “L” to “H” at time g. Then, the transfer signal φ1 shifts from “H” to “L” at time p, and shifts from “L” to “H” at time t. The same is repeated thereafter. That is, the period U(3) that begins at time f and the period U(4) that ends at time r represents one repetition cycle.

The transfer signal φ2 is “H” at time a, and shifts from “H” to “L” at time f. Then, the transfer signal φ2 shifts from “L” to “H” at time q, and shifts from “H” to “L” at time s. The same is repeated thereafter. That is, the period U(1) that begins at time a and the period U(3) that ends at time p represents one repetition cycle.

The transfer signal φ2 has a waveform that is shifted by the period U relative to the repeated waveform of the transfer signal φ1.

The transfer signal φ3 has a waveform shifted by ½ of the period U relative to that of the transfer signal φ1. The transfer signal φ4 has a waveform shifted by ½ of the period U relative to that of the transfer signal φ2.

The lighting signals φI1 and φI2 each have at least the following three levels of potentials: “H” (0 V), “L (“0”)”, and “L (“1”)”. For example, “L (“0”)” is −4.8 V, which is the sum of V (“0”) of −1.5 V that causes the laser diode LD to turn to the ON state of level “0” and −3.3 V that is applied to the setting thyristor S. Further, “L (“1”)” is −5.0 V, which is the sum of V (“1”) of −1.7 V that causes the laser diode LD to turn to the ON state of level “1” and −3.3 V that is applied to the setting thyristor S. To indicate the difference in optical intensity, in FIG. 18, the voltages of the lighting signals φI1 and φI2 are not illustrated based on their proportional relationship.

Now, the lighting signals φI1 and φI2 will be described with reference to the period U(3). The lighting signal φI1 is “H” at time f. Then, the lighting signal φI1 shifts from “H” to “L (“0”)” at time h. Then, the lighting signal φI1 shifts from “L (“0”)” to “L (“1”)” at time n, and shifts from “L (“1”)” to “H” at time o. Then, the lighting signal φI1 maintains its “H” level at time p. This is repeated for every period U. The period from time h to time n corresponds to the period σ representing the ON state of level “0”, and the period from time n to time o corresponds to the period τ representing the ON state of level “1”.

The lighting signal φI2 represents a waveform shifted relative to the lighting signal φI1 by ½ of the period U.

As will be described later, a period “H (+)” of a positive (+) potential may be provided instead of the period of “H” (0 V).

Next, the lighting control of the laser diodes LD will be described with reference to FIG. 18 taken together with FIG. 17. In FIG. 18, the ON state (each of the ON state of level “0” and the ON state of level “1”) of the laser diode LD is indicated by diagonal lines.

First, the lighting control of the set of odd-numbered laser diodes LD will be described.

Time a

At time a, when the start signal φs1 is “H”, the gate Gt1 of the transfer thyristor T1 goes to “H” (0 V). As described above, the threshold voltage of a thyristor is equal to the potential at the gate minus the forward potential Vd (1.5 V) of the p-n junction. Thus, the threshold voltage of the transfer thyristor T1 becomes −1.5 V.

The gate Gt3 of the transfer thyristor T3 turns to −1.5 V via the coupling diode D1. This causes the threshold voltage of the transfer thyristor T3 to become −3.0 V. The gate Gt5 of the transfer thyristor T5 goes to −3 V via the coupling diode D3. This causes the threshold voltage of the transfer thyristor T5 to become −4.5 V. Each of the transfer thyristors T numbered 7 or higher has its gate Gt set to “L (−3.3 V)”, which is the power supply potential Vgk1, via the corresponding power line resistor Rg, and thus has a threshold voltage of −4.8 V.

At this time, the transfer signal φ1 is “L” (−3.3 V), which is larger in absolute value than the threshold voltage (−1.5 V) of the transfer thyristor T1. Thus, the transfer thyristor T1 turns ON, and the gate Gt1 is maintained at “H” (0 V). Thyristors to which the transfer signal φ1 is transmitted, such as the transfer thyristor T5, have a threshold voltage (−4.5 V) that is larger in absolute value than the “L” (−3.3 V) level of the transfer signal φ1. Thus, these thyristors do not turn ON.

Meanwhile, since the transfer signal φ2 is “H” (0 V), the cathode and anode (the substrate 80) of each of the transfer thyristors T3, T7, and so on both become “H” (0 V), and thus these transfer thyristors do not turn ON.

Since the gate Gs1 of the setting thyristor S is connected to the gate Gt1, when the gate Gt1 goes to “H” (0 V), the gate Gs1 goes to “H” (0 V). Thus, the threshold voltage of the setting thyristor S becomes −1.5 V, causing the setting thyristor S to be set to a state in which the setting thyristor S is enabled to turn to the ON state (to be sometimes referred to as “turn-ON-enabled state” hereinafter).

At this time, the lighting signal φI1 is at the “L(“0”)” (−4.8V) level that causes the laser diode LD1 to light up in the ON state of level “0”. Thus, the laser diode LD1 goes to the ON state of level “0”. In this period in which the laser diode LD1 is in the ON state of level “0”, an oscillation delay and relaxation oscillation are caused to occur in the laser diode LD1 so that a steady state is subsequently achieved.

Time d

Times b and c are not related to the odd-numbered laser diodes LD.

At time d, as the lighting signal 411 goes to “L(“1”)” (−5.0 V) that causes the laser diode LD1 to light up in the ON state of level “1”, the laser diode LD1 goes to the ON state of level “1”. At this time, since an oscillation delay has been eliminated and relaxation oscillation has been attenuated for the laser diode LD1 in the period corresponding to the ON state of level “0”, fluctuations in optical intensity P or variations in optical energy are minimized.

Time e

At time e, as the lighting signal φ1 is shifted from “L (“1”)” to “H” (0 V), the cathode of the laser diode LD1 and the anode (the substrate 80) of the setting thyristor S1 both go to “H” (0 V). Thus, the setting thyristor S1 turns OFF, and the laser diode LD1 goes to the OFF state and is extinguished.

At this time, the lighting signal φ1 may be set to a potential (“H (+)” indicated by a broken line for the lighting signal φ1 in FIG. 18) that lies on the positive side with respect to “H” (0 V). By setting the lighting signal φI1 to the potential that is on the positive side, the electrons (carriers) are pulled away from each of the n-gate layer 82 and the p-gate layer 83 of the setting thyristor S, enabling faster extinguishing of the laser diode LD1.

Time f

At time f, as the transfer signal φ2 is shifted from “H” (0 V) to “L” (−3.3 V), the transfer thyristor T3 having a threshold voltage set to −3.0 V turns ON. Then, the gate Gt2 is set to 0 V. This causes the gate Gs3 of the setting thyristor S3 to become 0 V. Then, the threshold voltage of the setting thyristor S3 becomes −1.5 V, causing the setting thyristor S3 to be set to the turn-ON-enabled state.

Further, the gate Gt5 of the transfer thyristor T5 goes to −1.5 V via the coupling diode D3. Thus, the threshold voltage of the transfer thyristor T5 becomes −3 V.

At this time, the lighting signal φI1 is “H”. Thus, the setting thyristor S3 does not turn ON, and the laser diode LD3 does not light up.

At this time, the transfer thyristors T1 and T3 are both in their ON state.

Time g

At time g, the start signal φs1 is shifted from “H” to “L”, and the transfer signal φ1 is shifted from “L” to “H”.

Then, the anode and cathode of the transfer thyristor T1 both become “H”, causing the transfer thyristor T1 to turn OFF. Further, the gate Gt1 goes to “L” (−3.3 V), and the threshold voltage of the transfer thyristor T1 becomes −4.8 V.

That is, the transfer thyristor T1 goes to the OFF state, and thus the ON state is transferred (propagated) from the transfer thyristor T1 to the transfer thyristor T3.

Time h

At time h, as the lighting signal φ1 shifts from “H” to “L (“0”)”, the setting thyristor S3 turns ON, and the laser diode LD3 goes to the ON state of level “0”.

Time n

Times i to m are not related to the odd-numbered laser diodes LD.

At time n, as the lighting signal φ1 goes from “L (“0”)” to “L (“1”)”, the laser diode LD3 goes to the ON (lit) state of level “1”.

Time o

At time o, when the lighting signal φ1 is shifted from “L (“1”)” to “H” (0 V), as with time e, the cathode of the laser diode LD3 and the anode (the substrate 80) of the setting thyristor S3 both go to “H” (0 V). Thus, the setting thyristor S3 turns OFF, and the laser diode LD3 goes to the OFF state and is extinguished.

Time p

At time p, as the transfer signal φ1 goes from “H” to “L”, the transfer thyristor T5 having a threshold voltage set to −3 V turns ON. Then, the gate Gt5 and the gate Gs5 become 0 V, and the setting thyristor S5 is set to the turn-ON-enabled state.

Time q

At time q, the transfer signal φ2 is shifted from “L” to “H”. Then, the cathode and anode of the transfer thyristor T3 both become “H”, causing the transfer thyristor T3 to turn OFF. Further, the gate Gt3 goes to “L” (−3.3 V), and the threshold voltage of the transfer thyristor T3 becomes −4.8 V.

That is, the transfer thyristor T3 goes to the OFF state. Thus, the ON state is transferred (propagated) from the transfer thyristor T3 to the transfer thyristor T5.

Thereafter, in accordance with the transfer signals φ1 and φ2 and the lighting signal φI1, the same operation is repeated, and the odd-numbered laser diodes LD are lighting-controlled.

Next, the lighting control of the even-numbered laser diodes LD will be described.

Time a

At time a, the start signal φs2 is “L”, the transfer signal φ3 is “H”, and the transfer signal φ4 is “H”. At this time, as can be appreciated from FIG. 17, the gate Gt2 of the transfer thyristor T2 is “L” (−3.3 V), and thus the threshold voltage of the transfer thyristor T2 is −4.8 V. Likewise, the gate Gt2 of the setting thyristor S2 is also “L” (−3.3 V), and the threshold voltage of the setting thyristor S2 is −4.8 V.

Since the transfer signal φ3 is “H”, the cathode and anode (the substrate 80) of the transfer thyristor T3, to which the transfer signal φ3 is supplied, are both “H” (0 V), and thus the transfer thyristor T3 is in its OFF state.

Since the lighting signal φI2 is “H” (0 V), the setting thyristor S3 and the laser diode LD3 are in their OFF state.

The operations of the other thyristors (the transfer thyristors T and the setting thyristors S) not related to the laser diode LD of interest, and the laser diodes LD other than the laser diode LD of interest are the same as that described above with reference to the odd-numbered laser diodes LD, and thus will not be described further in detail.

Time b

When the start signal φs2 goes from “L” to “H”, the gate Gt2 of the transfer thyristor T2 and the gate Gs2 of the setting thyristor S2 both go to “H” (0 V), and the threshold voltage of the transfer thyristor T2 and the setting thyristor S2 becomes −1.5 V.

Then, the transfer signal φ3 supplied to the cathode of the transfer thyristor T2 shifts from “H” to “L” (−3.3 V), and the transfer thyristor T2 thus turns ON.

Since the lighting signal φI2 is maintained at the “H” level, the setting thyristor S2 does not turn ON, and the laser diode LD2 is also in its OFF state.

Time c

At time c, as the lighting signal φI2 shifts from “H” to “L (“0”)”, the setting thyristor S2 turns ON, and the laser diode LD2 goes to the ON state of level “0”.

Time i

Times d to h are not related to the lighting control of the even-numbered laser diodes LD.

At time i, as the lighting signal φI2 goes from “L (“0”)” to “L (“1”)”, the laser diode LD2 goes to the ON (lit) state of level “1”.

Time j

At time j, as the lighting signal φI2 goes from “L (“1”)” to “H”, the cathode of the laser diode LD2 and the anode (the substrate 80) of the setting thyristor S2 both go to “H”. Thus, the setting thyristor S2 turns OFF, and the laser diode LD2 goes to the OFF (extinguished) state.

The lighting period of the laser diode LD2 at level “1” (from time i to time j) is shifted backward by ½ of the period U relative to the lighting period of the laser diode LD1 at level “1” (from time d to time e). The lighting period of the laser diode LD2 at level “1” (from time i to time j) is shifted forward by ½ of the period U relative to the lighting period of the laser diode LD3 at level “1” (from time n to time o).

The subsequent lighting control of the even-numbered laser diodes LD is the same as the lighting control of the odd-numbered laser diodes LD already describe above, and thus will not be described further in detail.

When the laser diode LD is to be brought into the extinguished (unlit) state, this may be accomplished by retaining the lighting signal φI1 or the lighting signal φI2 at “H” (0 V). As a result, even when the threshold voltage of the setting thyristor S becomes −1.5 V, the setting thyristor S does not turn ON, and the laser diode LD is in the extinguished (unlit) state.

As described above, the gate terminals Gt of the transfer thyristors T are connected to each other via the corresponding coupling diodes D. Thus, a change in the potential of a given gate Gt causes a change in the potential of the gate Gt that is connected via the coupling diode D that is in forward biased condition to the gate Gt whose potential has changed. This in turn causes a change in the threshold voltage of the transfer thyristor T having the gate whose potential has changed. When the threshold voltage of the transfer thyristor T has a value higher than (a negative value smaller in absolute value than) the power supply potential Vgk1 or Vgk2 (“L” (−3.3 V)), the transfer thyristor T turns ON at the timing when the corresponding transfer signals φ (the transfer signals φ1 to φ4) shift from “H” (0 V) to “L” (−3.3 V).

At this time, the threshold voltage is −1.5 V for the setting thyristor S whose gate Gs is connected to the gate Gt of the transfer thyristor T that is in the ON state. Thus, when the corresponding lighting signal φI (the lighting signal φI1 or φI2) shifts from “H” (0 V) to “L” (“0”), the setting thyristor S turns ON and goes to the ON state of level “0”. When the lighting signal φI (the lighting signal φI1 or φI2) goes from “L (“0”)” to “L (“1”)”, the laser diode LD goes to the ON (lit) state of level “1”.

That is, as the transfer thyristor T is placed in the ON state, the target laser diode LD on which to perform lighting control is specified. At this time, the lighting signal φI (the lighting signal φI1 or φI2) at the “L (“0”)” level causes the setting thyristor S connected in series with the target laser diode LD to turn ON, and causes the laser diode LD to turn to the ON state of level “0”, and the lighting signal φI (the lighting signal φI1 or φI2) at the “L (“1”)” level causes the laser diode LD to turn to the ON (lit) state of level “1”.

The transfer thyristors T belonging to the set of odd-numbered laser diodes LD are driven by using the start signal φs1 and the transfer signals φ1 and φ2 to thereby perform lighting control of the set of odd-numbered laser diodes LD. The transfer thyristors T belonging to the set of even-numbered laser diodes LD are driven by using the start signal φs2 and the transfer signals φ3 and φ4 to thereby perform lighting control of the set of even-numbered laser diodes LD. Further, the period in which the set of odd-numbered laser diodes LD is lit at level “1”, and the period in which the set of odd-numbered laser diodes LD is lit at level “1” are alternated on the time axis. That is, multiple transfer paths are provided for individual sets of laser diodes LD, such as one transfer path for the set of odd-numbered laser diodes LD and one transfer path for the set of even-numbered laser diodes LD. Further, while the laser diodes LD belonging to one of the set of even-numbered laser diodes LDs and the set of odd-numbered laser diodes LD are lit at level “1”, the laser diodes LD belonging to the other one of the set of even-numbered laser diodes LDs and the set of odd-numbered laser diodes LD are lit at level “0”. This configuration causes lighting of the laser diodes LD at level “1” to be switched at short intervals of time. That is, the target laser diode LD that is to emit light is switched from one laser diode LD to another at high speed (fast response is achieved). For example, the target emitting laser diode LD is switched from one laser diode LD to another in a cycle equivalent to the length of time from time e to time j. If only the set of odd-numbered laser diodes LD is used, for example, the target laser diode LD is switched from one laser diode LD to another in a cycle equivalent to the length of time from the time e to time o.

The period σ of level “0” may be set in accordance with the state of oscillation delay or relaxation oscillation.

Although the above description is directed to a case where there are two (stages of) transfer paths, one for the set of odd-numbered laser diodes LD and one for the set of even-numbered laser diodes LD, three or more (stages of) transfer paths may be provided to achieve faster response.

The laser diode LD may be maintained in the ON state of level “0” at all times. That is, a direct-current voltage (direct-current bias) may be applied to the laser diode LD to place the laser diode LD in the ON state of level “0”.

The third exemplary embodiment may be applied to the second exemplary embodiment.

Although the laser component C according to the third exemplary embodiment uses the start signals φs1 and φs2, as with the laser component C according to the first exemplary embodiment, the laser component C according to the third exemplary embodiment may use the start diode SD.

Although the level represented as “0/1” is used in the above description, the level may be a combination of a level “m (m is an integer equal to or greater than 1)” and a level

Fourth Exemplary Embodiment

With the laser component C according to the third exemplary embodiment, the laser diodes LD are lighting-controlled by being switched in numerical order. In contrast, in a fourth exemplary embodiment, in the middle of lighting control, the next laser diode LD to be lighting-controlled is switched from one laser diode LD to another either in numerical order or in reverse numerical order.

FIG. 19 is an equivalent circuit diagram illustrating the circuit configuration of the laser component C according to the fourth exemplary embodiment and the signal generating circuit 110.

The laser component C according to the fourth exemplary embodiment is similar to the laser component C in the third exemplary embodiment except for the features described above. Accordingly, the following description will focus on differences, and like features are designated by the same reference signs and will not be described further in detail.

FIG. 19 depicts the portion of the laser component C corresponding to the laser diodes LD1 to LD9, the setting thyristors S1 to S9, and the transfer thyristors T1 to T9. The same description below will apply to the subsequently numbered diodes and thyristors.

Laser Generating Unit 10

The signal generating circuit 110 of the laser generating unit 10 includes the transfer signal generating unit 120, the lighting signal generating unit 140, the reference-potential supply unit 160, and the power-supply-potential supply unit 170.

The transfer signal generating unit 120 generates transfer signals φ1 φ2, φ3, φ4, φ5, φ6, φ7, φ8, and φ9 (to be referred to as transfer signal φ when not distinguished from each other), and start signals φs1, φs2, and φs3 (to be referred to as start signal φs when not distinguished from each other).

The lighting signal generating unit 140 supplies lighting signals φ1, φI2, and φI3 (to be referred to as lighting signal φI when not distinguished from each other).

The power-supply-potential supply unit 170 supplies power supply potentials Vgk1, Vgk2, and Vgk3 (to be referred to as power supply potential Vgk when not distinguished from each other).

The laser diodes LD, the setting thyristors S, and the transfer thyristors T of the laser component C are divided in numerical order into three sets. The three sets include a first set of laser diodes, setting thyristors, and transfer thyristors that are numbered 1+3×(n−1) (n is an integer equal to or greater than 1; the same applies hereinafter), such as the laser diodes LD1, LD4, LD7, and so on, a second set of laser diodes, setting thyristors, and transfer thyristors that are numbered 2+3×(n−1), such as the laser diodes LD2, LD5, LD8, and so on, and a third set of laser diodes, setting thyristors, and transfer thyristors that are numbered 3+3×(n−1), such as the laser diodes LD3, LD6, LD9, and so on.

The gates Gt of the transfer thyristors T in the first set are connected to the power supply line 71-1 via the corresponding power line resistors Rg. The power supply potential Vgk1 is supplied to the power supply line 71-1 from the power-supply-potential supply unit 170.

The gates Gt of the transfer thyristors T in the second set are connected to the power supply line 71-2 via the corresponding power line resistors Rg. The power supply potential Vgk2 is supplied to the power supply line 71-2 from the power-supply-potential supply unit 170.

The gates Gt of the transfer thyristors T in the third set are connected to the power supply line 71-3 via the corresponding power line resistors Rg. The power supply potential Vgk3 is supplied to the power supply line 71-3 from the power-supply-potential supply unit 170.

The cathodes (corresponding to the n-ohmic electrode 321 illustrated in FIGS. 5A and 5C) of the laser diodes LD in the first set are connected to the lighting signal line 75-1. The lighting signal φI1 is transmitted to the lighting signal line 75-1 from the lighting signal generating unit 140.

The cathodes of the laser diodes LD in the second set are connected to the lighting signal line 75-2. The lighting signal φI2 is transmitted to the lighting signal line 75-2 from the lighting signal generating unit 140.

The cathodes of the laser diodes LD in the third set are connected to the lighting signal line 75-3. The lighting signal φI3 is transmitted to the lighting signal line 75-3 from the lighting signal generating unit 140.

In each of the above-mentioned sets, the gates Gt of the transfer thyristors T are connected to each other by the coupling diode D and the coupling diode D′ that are connected in parallel in opposite directions. For example, in the first set, the gate Gt1 of the transfer thyristor T1 and the gate Gt4 of the transfer thyristor T4 are connected by the coupling diode D1 and the coupling diode D′1, and the gate Gt4 of the transfer thyristor T4 and the gate Gt7 of the transfer thyristor T7 are connected by the coupling diode D4 and the coupling diode D′4.

Further, the cathodes of the transfer thyristors T in the first set are connected to the transfer signal lines 72-1, 72-2, and 72-3 in a cyclic manner in the following numerical order: the transfer thyristors T1, T4, T7, and so on. The transfer signals φ1, φ2, and φ3 are respectively transmitted to the transfer signal lines 72-1, 72-2, and 72-3. The anode of the coupling diode D1 and the cathode of the coupling diode D′1, which are connected to the gate Gt1 of the transfer thyristor T1, are connected such that the start signal φs1 is transmitted to each of the anode and the cathode.

The cathodes of the transfer thyristors T in the second set are connected to transfer signal lines 72-4, 72-5, and 72-6 in a cyclic manner in the following numerical order: the transfer thyristors T2, T5, T8, and so on. The transfer signals φ4, φ5, and φ6 are respectively transmitted to the transfer signal lines 72-4, 72-5, and 72-6. The anode of the coupling diode D2 and the cathode of the coupling diode D′2, which are connected to the gate Gt2 of the transfer thyristor T2, are connected such that the start signal φs2 is transmitted to each of the anode and the cathode.

Likewise, the cathodes of the transfer thyristors T in the third set are connected to transfer signal lines 72-7, 72-8, and 72-9 in a cyclic manner in the following numerical order: the transfer thyristors T3, T6, T9, and so on. The transfer signals φ7, φ8, and φ9 are respectively transmitted to the transfer signal lines 72-7, 72-8, and 72-9. The anode of the coupling diode D3 and the cathode of the coupling diode D′3, which are connected to the gate Gt3 of the transfer thyristor T3, are connected such that the start signal φs3 is transmitted to each of the anode and the cathode.

In FIG. 19, symbols representing individual signals (such as φ1 representing the transfer signal φ1), and symbols representing the terminals to which the corresponding signals are supplied (such as φ1 representing the φ1 terminal) are the same, and thus symbols representing terminals are not illustrated.

Further, the current-limiting resistors are labeled R and RI.

Operation of Laser Generating Unit 10

FIG. 20 is a timing chart illustrating the operation of the laser generating unit 10 according to the fourth exemplary embodiment.

The timing chart illustrated in FIG. 20 represents lighting control of nine laser diodes, the laser diodes LD1 to LD9, of the laser component C.

It is assumed in FIG. 20 that time elapses in alphabetical order from time a to time q (times a to o differ from times a to w illustrated in FIG. 18). The laser diode LD1, the laser diode LD2, and the laser diode LD3 respectively go to the ON (lit) state of level “1” in the period U(1) (from time a to time c), the period U(2) (from time c to time e), and the period U(3) (from time e to time i). The laser diodes LD that are numbered 4 or higher are subsequently lighting-controlled in the same manner. It is assumed in this case that the periods U(1), U(2), U(3), and so on are of the same length, and will be referred to as period U when not distinguished from each other.

The transfer signals φ1 to φ3 respectively transmitted to the φ1 to φ3 terminals corresponding to the first set, and the start signal φs1 transmitted to the φ1 terminal corresponding to the first set each have two levels of potentials, “H” (0 V) and “L” (−3.3 V). In the following description, “H” (0 V) and “L” (−3.3 V) will be sometimes abbreviated as “H” and “L”, respectively.

The transfer signal φ1 is “L” at time a, shifts from “L” to “H” at time f, and shifts from “H” to “L” at time o.

The transfer signal φ2 is “H” at time a, shifts from “H” to “L” at time e, and shifts from “L” to “H” at time l.

The transfer signal φ3 is “H” at time a, shifts from “H” to “L” at time k, and shifts from “L” to “H” at time p.

The transfer signals φ1 to φ3 repeat their respective waveforms from time a to time q.

Meanwhile, the start signal φs1 is “H” at time a, and shifts from “H” to “L” at time f when the transfer signal φ1 shifts from “L” to “H”. Thereafter, the start signal φs1 maintains its “L” level.

The transfer signals φ4, φ5, and φ6 corresponding to the second set are shifted backward on the time axis by the period U relative to the transfer signals φ1, φ2, and φ3 corresponding to the first set.

The transfer signals φ7, φ8, and φ9 corresponding to the third set are shifted backward on the time axis by the period U relative to the transfer signals φ4, φ5, and φ6 corresponding to the second set.

Each of the transfer signals φ1 to φ9 has a period of “L” level whose length is equivalent to the length of time represented as follows: 3× period U+period of time from time k to time l, and has a period of “H” level whose length is equivalent to the length of time represented as follows: 6× period U−period of time from time k to time l. The respective periods of “L” level (e.g., from time g to time h) of the transfer signals φ1 and φ2 partially overlap. The same applies to the transfer signals φ2 and φ3, and the same applies to the transfer signals φ3 and φ1. Likewise, the same applies to the other transfer signals φ.

The lighting signal φI1 is “L (“1”)” at time a, shifts from “L (“1”)” to “L (“0”)” at time c, and shifts from “L (“0”)” to “H” at time g. Then, the lighting signal φI1 shifts from “H” to “L (“0”)” at time h, and shifts from “L (“0”)” to “L (“1”)” at time i. The lighting signal φI1 repeats its waveform from time a to time i.

That is, the lighting signal φI1 causes the corresponding laser diode LD to become the ON state of level “0” in a period (a period σ1 corresponding to the period of time from time h to time i) that precedes the period U in which the laser diode LD is in the ON (lit) state of level “1” (the period τ of the “L(“1”)” level), and causes the laser diode LD to become the ON state of level “0” in a period (a period σ2 corresponding to the period of time from time c to time g) that follows the above-mentioned period U. Further, the lighting signal φI1 has, between the period σ2 and period σ1, a period in which the lighting signal φI1 is at “H”.

The lighting signal φI1 repeats its waveform in a cycle of 3× period U.

The lighting signal φI2 is shifted backward by the period U on the time axis relative to the lighting signal φI1. Likewise, the lighting signal φI3 is shifted backward by the period U on the time axis relative to the lighting signal φI2.

The operation of the laser component C will be described below in time sequence. Since the operation of thyristors (the transfer thyristor T and the setting thyristor S) has been described above in detail with reference to the first exemplary embodiment, the following description will focus on differences from the first exemplary embodiment.

Time a

At time a, the start signal φs1 is “H” (0 V), and thus the threshold voltage of the transfer thyristor T1 is −1.5 V. At this time, the transfer signal φ1 is “L” (−3.3 V), and the transfer thyristor T1 thus turns ON. The threshold voltage of the setting thyristor S1 is also −1.5 V. Since the lighting signal φI1 is “L (“1”)”, the laser diode LD1 goes to the ON (lit) state of level “1”.

At this time, the transfer thyristor T4 connected by the coupling diode D1 that is in forward biased condition has a threshold voltage of −3.0 V.

Likewise, the transfer thyristor T2 also turns ON.

Since the start signal φs3 is “H”, the threshold voltage of the transfer thyristor T3 is −1.5 V. However, since the transfer signal φ7 is “H”, the transfer thyristor T3 is in its OFF state.

Time b

At time b, as the lighting signal φI2 shifts from “H” to “L (“0”)”, the laser diode LD2 goes to the ON state of level “0”.

Time c

At time c, the transfer signal φ7 shifts from “H” to “L”, and thus the transfer thyristor T3 turns ON.

At this time, as the lighting signal φI1 shifts from “L (“1”)” to “L (“0”)”, the laser diode LD1 goes to the ON state of level “0”.

Further, as the lighting signal φI2 shifts from “L (“0”)” to “L (“1”)”, the laser diode LD2 goes to the ON (lit) state of level “1”.

Time d

At time d, as the lighting signal φI3 shifts from “H” to “L (“0”)”, the laser diode LD3 goes to the ON state of level

Time e

At time e, the transfer signal φ2 shifts from “H” to “L” (−3.3 V). Then, the transfer thyristor T4 whose threshold voltage has become −3.0 V turns ON. Then, the threshold voltage of the transfer thyristor T7 goes to −3.0 V via the coupling diode D4.

The lighting signal φI2 goes from “L (“1”)” to “L (“0”)”, and the laser diode LD2 goes from the OFF state to the ON state of level “0”.

Further, when the lighting signal φI3 shifts from “L (“0”)” to “L (“1”)”, the laser diode LD3 shifts from the ON state of level “0” to the ON (lit) state of level “1”.

Time f

At time f, as the transfer signal φ1 goes from “L” to “H”, the cathode of the transfer thyristor T1 goes to “H”, and thus the transfer thyristor T1 turns OFF. At this time, the start signal φs1 goes from “H” to “L”.

At this time, the gate Gt1 of the transfer thyristor T1 is connected to the gate Gt4 of the transfer thyristor T4 by the coupling diode D′1. The coupling diode D1 is reverse biased, whereas the coupling diode D′1 is forward biased, and the gate Gt1 becomes −1.5 V. This causes the threshold voltage of the transfer thyristor T1 to become −3.0 V. That is, when the transfer thyristor T4 goes to the ON state, the threshold voltage of the transfer thyristors T1 and T7 becomes −3.0 V due to the coupling diodes D1 and D′1 that are connected in parallel and biased in opposite directions.

Although the gate Gs1 of the setting thyristor S1 also becomes −1.5 V, since the lighting signal 411 is “L (“0”)”, the ON state is maintained.

Time g

At time g, as the lighting signal φI1 goes from “L (“0”)” to “H”, the laser diode LD1 goes from the ON state of level “0” to the OFF state. As a result, the setting thyristor S1 also goes from the ON state to the OFF state. Then, the threshold voltage of the setting thyristor S1 becomes −3.0 V.

Time h

At time h, as the lighting signal φI1 goes from “H” to “L (“0”)”, the setting thyristor S4 with a threshold voltage of −1.5 V turns ON, and the laser diode LD4 goes from the OFF state to the ON state of level “0”.

Time i

At time i, as the transfer signal φ5 goes from “H” to “L”, the transfer thyristor T5 turns ON.

Further, when the lighting signal φI1 goes from “L (“0”)” to “L (“1”)”, the laser diode LD4 goes from the ON state of level “0” to the ON (lit) state of level “1”.

Thereafter, at time k, as the transfer signal φ3 shifts from “H” to “L”, the transfer thyristor T7 turns ON.

In this way, the laser diodes LD are switched to the ON (lit) state of level “1” in numerical order.

That is, the transfer signals φ1, φ2, and φ3 are made to shift from “H” to “L” in this order in a cyclic manner. This causes the ON state to be transferred (propagated) for the first set of transfer thyristors T (the transfer thyristors T1, T4, T7, and so on) in numerical order. The same applies to the other sets of transfer thyristors T.

Further, the transfer signals φ1, φ4, φ7, φ2, φ5, φ8, φ3, φ6, and φ9 are made to shift from “H” to “L” in this order in a cyclic manner. This causes the ON state to be transferred (propagated) for the transfer thyristors T in numerical order.

Then, depending on the potential (“L (“0”)” or “L (“1”)”) of the lighting signal φI in a state in which the threshold voltage of the setting thyristor S corresponding to the transfer thyristor T that is in the ON state has become small in absolute value, the laser diode LD turns to either the ON state of level “0” or the ON (lit) state of level “1”.

In the above-mentioned case, the laser diodes LD also turn to the ON (lit) state in numerical order. Further, the period σ1 in which the laser diode LD is placed in the ON state of level “0” is provided before the laser diode LD is brought into the ON (lit) state of level “1”. This helps to eliminate the influence of an oscillation delay or relaxation oscillation occurring in the laser diode LD.

If the OFF state is to be maintained, this may be accomplished by keeping the potential of the lighting signal φI at “H”.

There are times when, in the middle of switching the laser diodes LD into the ON (lit) state in numerical order, it is desired to switch the laser diodes LD into the ON (lit) state in reverse numerical order. For example, in FIG. 20, after the ON (lit) state is propagated for the laser diodes LD in the order LD1, LD2, LD3, LD4, and LD5, the ON (lit) state is then propagated for the laser diodes LD in reverse numerical order as LD4, LD3, LD2, and LD1.

In this case, at time k, the lighting signal φI1 is not shifted from “L (“0”)” to “L (“1”)”, and the lighting signal φI3 is not shifted from “L (“0”)” to “L (“1”)”. Instead, these lighting signals are maintained at “L (“0”)”. This causes the laser diode LD4 to go from the ON state of level “0” to the ON (lit) state of level “1” again. Since the lighting signal φI1 is set at “L (“0”)” at time k, the influence of an oscillation delay or relaxation oscillation is eliminated.

Further, for example, the transfer signals φ are adjusted so that the transfer signals φ7, φ4, φ1, φ9, φ6, φ3, φ8, φ5, and φ2 shift from “H” to “L” in this order in a cyclic manner, such that the transfer signal φ7 shifts from “H” to “L” at time k, the transfer signal φ4 shifts from “H” to “L” at time m, and the transfer signal φ1 shifts from “H” to “L” at time n. This allows the ON state to be propagated for the transfer thyristors T in their reverse numerical order, and also allows the ON (lit) state to be propagated for the laser diodes LD in their reverse numerical order.

As described above, with the laser component C according to the fourth exemplary embodiment, the laser diodes LD are divided into at least three sets of laser diodes LD, with a transfer path provided for each of these sets. In this way, the period corresponding to the ON state of level “0” is provided before and after the period corresponding to the ON (lit) state of level “1”. Further, each set of laser diodes is driven by using the transfer signals φ of at least three phases to enable switching of the direction of transfer. Therefore, irrespective of whether the ON (lit) state of level “1” is switched in a transfer direction corresponding to the direction of the array of the laser diodes LD (in numerical order) or in a transfer direction corresponding to the direction opposite to this direction (in reverse numerical order), the laser diodes LD are not subject to the influence of an oscillation delay or relaxation oscillation.

In the above-mentioned example, in the middle of propagating the ON (lit) state of level “1” for the laser diodes LD in numerical order, the ON (lit) state of level “1” is caused to be propagated in reverse numerical order. In this regard, this propagation may be controlled such that propagation in the direction of the array (in numerical order) and propagation in the opposite direction (in reverse numerical order) are alternately repeated.

That is, after the wavelength λ is scanned (swept) from shorter to longer wavelengths, the wavelength λ may be conversely scanned (swept) from longer to shorter wavelengths. Further, in the middle of scanning (sweeping) the wavelength λ from shorter to longer wavelengths, or in the middle of scanning (sweeping) the wavelength λ from longer to shorter wavelengths, the wavelength λ may be scanned in reverse direction. Alternatively, in the middle of scanning (sweeping), the target laser diode LD may be fixed to one with a specific wavelength λ.

The laser component C according to the fourth exemplary embodiment is fabricated by partially modifying the fabrication method according to the first exemplary embodiment illustrated in FIGS. 10A to 10F. Thus, the fabrication method will not be described further in detail.

The method of propagating the ON state in reverse direction described above with reference to the fourth exemplary embodiment may be employed for the first exemplary embodiment or the second exemplary embodiment.

Although the laser component C according to the fourth exemplary embodiment uses the start signals φs1, φs2, and φs3, as with the laser component C according to the first exemplary embodiment, the laser component C according to the fourth exemplary embodiment may use the start diode SD.

Fifth Exemplary Embodiment

The laser diode LD, which is an example of a light emitting device in the laser component C of the laser generating unit 10 according to each of the first to fourth exemplary embodiments, has the light emitting layer 87 sandwiched by the p-anode layer 86 and the n-cathode layer 88 that act as cladding layers.

However, instead of the laser diode LD, the light emitting device used may be a vertical cavity surface emitting laser (VCSEL) that has the light emitting layer 87 sandwiched by two distributed Bragg reflectors (DBRs) (to be referred to as DBR layers hereinafter), and emits light in a direction perpendicular to the light emitting layer 87. The vertical cavity surface emitting laser VCSEL oscillates when the two DBR layers have a reflectance equal to or higher than, for example, 99%. The emission wavelength is determined by the thickness of the light emitting layer 87 sandwiched by the two DBR layers.

The plan layout and cross-section of the laser component C according to the fifth exemplary embodiment are similar to those of the laser component C according to the first exemplary embodiment illustrated in FIGS. 4A and 4B. Accordingly, the laser diode LD in the arrangement according to the first exemplary embodiment may be replaced by the vertical cavity surface emitting laser VCSEL.

A description will be given below of the island 301 formed by stacking the vertical cavity surface emitting laser VCSEL and the setting thyristor S, which represents a feature different from the first exemplary embodiment.

FIG. 21 is a detailed illustration of the island 301 formed by stacking the setting thyristor S and the vertical cavity surface emitting laser VCSEL in the laser component C according to the fifth exemplary embodiment.

With the laser component C according to the fifth exemplary embodiment, the p-anode layer 86 and the n-cathode layer 88 are constructed as DBR layers. The p-anode layer 86 includes the current confinement layer 86 b. That is, the p-anode layer 86 is formed by stacking the lower p-anode layer 86 a, the current confinement layer 86 b, and the upper p-anode layer 86 c in this order, with the lower p-anode layer 86 a and the upper p-anode layer 86 c being constructed as DBR layers.

Laser is emitted in a direction perpendicular to the substrate 80. Thus, the n-ohmic electrode 321 has an opening in the central portion.

The lower p-anode layer 86 a, the upper p-anode layer 86 c, and the n-cathode layer 88 will be sometimes respectively referred to as lower p-anode (DBR) layer 86 a, upper p-anode (DBR) layer 86 c, and n-cathode (DBR) layer 88. In FIG. 21, these layers are designated as pDBR and nDBR.

Each DBR layer is formed by a combination of a low refractive index layer of, for example, Al_(0.9)Ga_(0.1)As with a high Al composition, and a high refractive index layer of, for example, Al_(0.2)Ga_(0.8)As with a low Al composition. The film thickness (optical path length) of each of the low refractive index layer and the high refractive index layer is set to, for example, 0.25 (¼) of the center wavelength. The composition ratio of Al in each of the low refractive index layer and the high refractive index layer may be varied in the range of 0 to 1.

The film thickness (optical path length) of the current confinement layer 86 b is determined by the specific structure employed. If the priority is extraction efficiency or process reproducibility, the film thickness (optical path length) of the current confinement layer 86 b is desirably set to an integer multiple of the film thickness (optical path length) of each of the low refractive index layer and the high refractive index layer that form the DBR layers, for example, to 0.75 (¾) of the center wavelength. If this integer is an odd value, it is desirable to sandwich the current confinement layer 86 b by a high refractive index layer and a high refractive index layer. If this integer is an even value, it is desirable to sandwich the current confinement layer 86 b by a high refractive index layer and a low refractive index layer. That is, the current confinement layer 86 b is desirably disposed so as to minimize periodic perturbation of the refractive index due to the DBR layers. Conversely, if it is desired to reduce the influence of the oxidized portion (refractive index or distortion), it is desirable that the current confinement layer 86 b have a film thickness of several tens nm, and be inserted at the nodes of standing waves that appear in the DBR layers.

The laser component C according to the fifth exemplary embodiment is fabricated by partially modifying the fabrication method according to the first exemplary embodiment illustrated in FIGS. 10A to 10F.

For example, a patterned substrate formed by providing the surface of the substrate 80 with irregularities (steps) is used, with the speed of growth of each semiconductor layer changing with the depth or width of the steps. This causes the thickness of the light emitting layer 87 to vary, allowing multiple vertical cavity surface emitting lasers VCSEL forming the laser device array to emit different wavelengths.

In the semiconductor laminate forming step illustrated in FIG. 10A, the lower p-anode layer 86 a and the upper p-anode layer 86 c of the p-anode layer 86, and the n-cathode layer 88 are formed as DBR layers. A portion of a semiconductor layer, such as a portion of the lower p-anode layer 86 a or the upper p-anode layer 86 c of the p-anode layer 86, or a portion of the n-cathode layer 88, may be formed as a DBR layer.

The laser component C according to the fifth exemplary embodiment may be fabricated as described above.

Instead of the tunnel junction layer 85, a group III-V compound layer with a metal-like conductivity may be used.

The vertical cavity surface emitting laser VCSEL may be employed for the first to fourth exemplary embodiments.

Sixth Exemplary Embodiment

With the laser component C according to each of the first to fifth exemplary embodiments, the laser diode LD is disposed over the setting thyristor S disposed over the substrate 80.

In the sixth exemplary embodiment, the laser diode LD is disposed over the substrate 80, and the setting thyristor S is disposed over the laser diode LD.

Since the sixth embodiment is otherwise of the same configuration as the first exemplary embodiment, the following describes the island 301 formed by stacking the setting thyristor S and the laser diode LD, which represents a feature different from the first exemplary embodiment.

FIG. 22 is a detailed illustration of the island 301, which is formed by stacking the setting thyristor S and the laser diode LD, of the laser component C according to the sixth exemplary embodiment. The same reference signs as those used in the first exemplary embodiment are used in the sixth exemplary embodiment.

The p-anode layer 86, the light emitting layer 87, the n-cathode layer 88, the tunnel junction layer 85, the p-anode layer 81, the n-gate layer 82, the p-gate layer 83, and the n-cathode layer 84 are stacked in this order over the substrate 80. The p-anode layer 86 and the n-cathode layer 88 are cladding layers. Accordingly, the p-anode layer 86 and the n-cathode layer 88 will be sometimes respectively referred to as p-anode (cladding) layer 86 and n-cathode (cladding) layer 88.

The p-anode layer 86 includes the lower p-anode layer 86 a, the current confinement layer 86 b, and the upper p-anode layer 86 c.

As with the n-cathode layer 88 according to the first exemplary embodiment, the n-cathode layer 88 is made up of the lower n-cathode layer 88 a and the upper diffraction grating layer 88 b. The upper diffraction grating layer 88 b, which is a portion with diffraction gratings (banded irregularities) formed at predetermined pitches, is contiguous with the lower n-cathode layer 88 a.

Although the tunnel junction layer 85 may be stacked over the upper diffraction grating layer 88 b, desirably, an n-cathode layer with a refractive index different from that of the n-cathode layer 88 is disposed over the upper diffraction grating layer 88 b, and the tunnel junction layer 85 is stacked over this n-cathode layer.

That is, as with the laser diode LD according to the first exemplary embodiment illustrated in FIGS. 5A to 5C, the laser component C according to the sixth exemplary embodiment emits light along the surface of the substrate 80.

As described above with reference to the fifth exemplary embodiment, the p-anode layer 86 and the n-cathode layer 88, which are cladding layers, may be constructed as DBR layers. In this case, the laser diode LD becomes a vertical cavity surface emitting laser VCSEL. The vertical cavity surface emitting laser VCSEL with the p-anode layer 86 and the n-cathode layer 88 formed as DBR layers emits light in a direction perpendicular to the substrate 80.

Instead of the tunnel junction layer 85, a group III-V compound layer with a metal-like conductivity may be used.

The configuration in which the laser diode LD or the vertical cavity surface emitting laser VCSEL is disposed over the substrate 80, and the setting thyristor S is disposed over the laser diode LD or the vertical cavity surface emitting laser VCSEL, may be employed for the first to fourth exemplary embodiments.

With the laser component C of the laser generating unit 10 according to the first to sixth exemplary embodiments, the conductivity types of the light emitting device (the laser diode LD or the vertical cavity surface emitting laser VCSEL) and the thyristors (the transfer thyristor T and the setting thyristor S) may be reversed, and the polarity of the circuit may be changed. That is, the anode common configuration may be changed to a cathode common configuration.

The transfer thyristor T and the setting thyristor S according to each exemplary embodiment may have a structure other than the four-layer p-n-p-n structure as long as the structure achieves the functions of the transfer thyristor T and the setting thyristor S in each exemplary embodiment. For example, the structure employed may be a p-i-n-i-n structure, a p-i-p-i-n structure, an n-p-i-p structure, or a p-n-i-n structure that exhibits characteristics of a thyristor. In this case, one of the i-layer, the n-layer, and the i-layer sandwiched between “p” and “n” of the p-i-n-i-n-structure, or one of the n-layer and the i-layer sandwiched between “p” and “n” of the p-n-i-n structure may serve as a gate layer, and an n-ohmic electrode disposed over the gate layer may serve as a terminal corresponding to the gate Gt (the gate Gs). Alternatively, one of the i-layer, the p-layer, and the i-layer sandwiched between “n” and “p” of the p-i-p-i-n structure, or one of the p-layer and the i-layer sandwiched between “n” and “p” of the n-p-i-p structure may serve as a gate layer, and a p-ohmic electrode disposed over the gate layer may serve as a terminal corresponding to the gate Gt (the gate Gs).

The foregoing description is directed to use of a p-type GaAs substrate as an example of the substrate 80. The following describes an example of various semiconductor layers (the semiconductor laminate formed in the semiconductor laminate forming step illustrated in FIG. 10A) that are formed when other substrates are used.

First, an example of a semiconductor laminate formed when a GaN substrate is used will be described below.

The p-anode layer 81 is a p-type Al_(0.9)GaN layer with an impurity concentration of 1×10¹⁸/cm³, for example. The Al composition may be varied in the range from 0 to 1.

The n-gate layer 82 is an n-type Al_(0.9)GaN layer with an impurity concentration of 1×10¹⁷/cm³, for example. The Al composition may be varied in the range from 0 to 1.

The p-gate layer 83 is a p-type Al_(0.9)GaN layer with an impurity concentration of 1×10¹⁷/cm³, for example. The Al composition may be varied in the range from 0 to 1.

The n-cathode layer 84 is an n-type Al_(0.9)GaN layer with an impurity concentration of 1×10¹⁸/cm³, for example. The Al composition may be varied in the range from 0 to 1.

The tunnel junction layer 85 is formed by a junction (see FIG. 10B) of the n⁺⁺ layer 85 a doped with a high concentration of n-type impurities and the p⁺⁺ layer 85 b doped with a high concentration of p-type impurities. The n⁺⁺ layer 85 a and the p⁺⁺ layer 85 b each have a high impurity concentration of, for example, 1×10²⁰/cm³. The impurity concentration of a typical junction is on the order of 10¹⁷/cm³ to 10¹⁸/cm³. Examples of combinations of the n⁺⁺ layer 85 a and the p⁺⁺ layer 85 b (to be referred to as n⁺⁺ layer 85 a/p⁺⁺ layer 85 b hereinafter) include n⁺⁺GaN/p⁺⁺GaN, n⁺⁺GaInN/p⁺⁺GaInN, and n⁺⁺AlGaN/p⁺⁺AlGaN. These combinations may be changed with each other.

The p-anode (cladding) layer 86 is formed by stacking the lower p-anode (cladding) layer 86 a, the current confinement layer 86 b, and the upper p-anode (cladding) layer 86 c in this order (see FIG. 10C).

The lower p-anode (cladding) layer 86 a and the upper p-anode (cladding) layer 86 c are, for example, p-type Al_(0.9)GaN layers with an impurity concentration of 1×10¹⁸/cm³. The Al composition may be varied in the range from 0 to 1.

It is difficult to use an oxidized confinement layer as a current confinement layer on a GaN substrate. Accordingly, it is desired to employ a structure in which a tunnel junction layer or metal-like-conductivity group III-V compound layer is disposed in the portion corresponding to the current-passing portion α, or use the n-cathode (cladding) layer 88 formed as a ridge or buried structure. Alternatively, it is also effective to use ion implantation as a current confinement method.

The light emitting layer 87 has a quantum well structure formed by an alternate stack of well and barrier layers. The well layer is made of, for example, GaN, InGaN, or AlGaN, and the barrier layer is made of, for example, AlGaN or GaN. The light emitting layer 87 may be made of quantum lines (quantum wires) or quantum boxes (quantum dots). The light emitting layer 87 may be formed by a combination of a well layer, a barrier layer, and spacer layers disposed above and below these layers. The light emitting layer 87 may be formed by, for example, a resonator structure.

The n-cathode (cladding) layer 88 is an n-type Al_(0.9)GaN layer with an impurity concentration of 1×10¹⁸/cm³, for example. The Al composition may be varied in the range from 0 to 1.

Next, an example of a semiconductor laminate formed when an InP substrate is used will be described below.

The p-anode layer 81 is a p-type InGaAsP layer with an impurity concentration of 1×10¹⁸/cm³, for example. The Ga composition and the Al composition may be varied in the range from 0 to 1.

The n-gate layer 82 is an n-type InGaAsP layer with an impurity concentration of 1×10¹⁷/cm³, for example. The Ga composition and the Al composition may be varied in the range from 0 to 1.

The p-gate layer 83 is a p-type InGaAsP layer with an impurity concentration of 1×10¹⁷/cm³, for example. The Ga composition and the Al composition may be varied in the range from 0 to 1.

The n-cathode layer 84 is an n-type InGaAsP layer with an impurity concentration of 1×10¹⁸/cm³, for example. The Ga composition and the Al composition may be varied in the range from 0 to 1.

The tunnel junction layer 85 is formed by a junction (see FIG. 10B) of the n⁺⁺ layer 85 a doped with a high concentration of n-type impurities and the p⁺⁺ layer 85 b doped with a high concentration of p-type impurities. The n⁺⁺ layer 85 a and the p⁺⁺ layer 85 b each have a high impurity concentration of, for example, 1×10²⁰/cm³. The impurity concentration of a typical junction is on the order of 10¹⁷/cm³ to 10¹⁸/cm³. Examples of combinations of the n⁺⁺ layer 85 a and the p⁺⁺ layer 85 b (to be referred to as n⁺⁺ layer 85 a/p⁺⁺ layer 85 b hereinafter) include n⁺⁺InP/p⁺⁺InP, n⁺⁺InAsP/p⁺⁺InAsP, n⁺⁺InGaAsP/p⁺⁺InGaAsP, and n⁺⁺InGaAsPSb/p⁺⁺InGaAsPSb. These combinations may be changed with each other.

The p-anode (cladding) layer 86 is formed by stacking the lower p-anode (cladding) layer 86 a, the current confinement layer 86 b, and the upper p-anode (cladding) layer 86 c in this order (see FIG. 10C).

The lower p-anode (cladding) layer 86 a and the upper p-anode (cladding) layer 86 c are, for example, p-type InGaAsP layers with an impurity concentration of 1×10¹⁸/cm³. The Ga composition and the Al composition may be varied in the range from 0 to 1.

It is difficult to use an oxidized confinement layer as a current confinement layer on an InP substrate. Accordingly, it is desired to employ a structure in which a tunnel junction layer or metal-like-conductivity group III-V compound layer is disposed in the portion corresponding to the current-passing portion α, or use the n-cathode (cladding) layer 88 formed as a ridge or buried structure. Alternatively, it is also effective to use ion implantation as a current confinement method.

The light emitting layer 87 has a quantum well structure formed by an alternate stack of well and barrier layers. The well layer is made of, for example, InAs, InGaAsP, AlGaInAs, or GaInAsPSb, and the barrier layer is made of, for example, InP, InAsP, InGaAsP, or AlGaInAsP. The light emitting layer 87 may be made of quantum lines (quantum wires) or quantum boxes (quantum dots). The light emitting layer 87 may be formed by a combination of a well layer, a barrier layer, and spacer layers disposed above and below these layers. The light emitting layer 87 may be formed by, for example, a resonator structure.

The n-cathode (cladding) layer 88 is an n-type InGaAsP layer with an impurity concentration of 1×10¹⁸/cm³, for example. The Ga composition and the Al composition may be varied in the range from 0 to 1.

These semiconductor layers are stacked by, for example, metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) to form a semiconductor laminate.

The present invention is not limited to the above exemplary embodiments but may be practiced with various modifications without departing from the scope of the invention. The exemplary embodiments may be applied to each other.

Further, the configuration according to the exemplary embodiments of the invention may be also employed for a p-type/n-type/i-type layer made of an organic material.

The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents. 

What is claimed is:
 1. A laser component comprising: a plurality of laser devices arranged in an array to include different emission wavelengths; and a driving unit that, while switching each of the laser devices along the array into a turn-ON-enabled state that enables the laser device to turn to an ON state, brings one of the laser devices in the turn-ON-enabled state into an ON state.
 2. The laser component according to claim 1, wherein the laser devices are arranged such that their emission wavelengths are in increasing or decreasing order in a direction of the array.
 3. The laser component according to claim 1, wherein the driving unit switches each of the laser devices into the turn-ON-enabled state in a direction of the array and in a direction opposite to the direction of the array.
 4. The laser component according to claim 2, wherein the driving unit switches each of the laser devices into the turn-ON-enabled state in a direction of the array and in a direction opposite to the direction of the array.
 5. The laser component according to claim 1, wherein each of the laser devices has an ON state of level “m (m is an integer equal to or greater than 1)”, an ON state regarded to be of level “0”, and an OFF state of level “0”, and wherein the laser devices are divided into a plurality of sets of laser devices, and the driving unit includes a plurality of transfer paths that sequentially transfer the turn-ON-enabled state for each of the sets such that while laser devices included in one of the sets is in the ON state of level “m”, laser devices included in another one of the sets are placed in the ON state of level “0”.
 6. The laser component according to claim 1, wherein the driving unit includes a plurality of setting thyristors, each of the setting thyristors being stacked together with a corresponding one of the laser devices such that a tunnel junction layer or a group III-V compound layer having a metal-like conductivity is interposed between the setting thyristor and the laser device, each of the setting thyristors setting, upon turning ON, the corresponding one of the laser devices to the turn-ON-enabled state.
 7. A laser generating apparatus comprising: a laser component including a plurality of laser devices arranged in an array to include different emission wavelengths, and a driving unit that, while switching each of the laser devices along the array into a turn-ON-enabled state that enables the laser device to turn to an ON state, brings one of the laser devices in the turn-ON-enabled state into an ON state; and a controller that supplies a transfer signal and a lighting signal to the driving unit of the laser component, the transfer signal causing the turn-ON-enabled state to be transferred along the array of the laser devices, the lighting signal causing one of the laser devices in the turn-ON-enabled state to turn ON.
 8. An optical coherence tomograph comprising: a laser generator that includes a laser component including a plurality of laser devices arranged in an array to include different emission wavelengths, and a driving unit that, while switching each of the laser devices along the array into a turn-ON-enabled state that enables the laser device to turn to an ON state, brings one of the laser devices in the turn-ON-enabled state into an ON state, and a controller that supplies a transfer signal and a lighting signal to the driving unit of the laser component, the transfer signal causing the turn-ON-enabled state to be transferred along the array of the laser devices, the lighting signal causing one of the laser devices in the turn-ON-enabled state to turn ON; an optical splitting and combining unit that performs splitting and combining of light; a reference-light reflecting unit that reflects one of two beams of light split by the optical splitting and combining unit; and a photo-detection unit that detects interference light produced by combination of two reflected beams of light. 